Why Do Plants and Animals Change?

Let's explore why living things, especially those raised by humans, change over time. We'll look at:

What causes these changes (variability).

How habits and using (or not using) body parts make a difference.

How changing one part can affect another (correlated variation).

How traits are passed down (inheritance).

What makes domestic plants and animals special.

Why it's hard to tell varieties and species apart.

Where domestic varieties originally came from (one ancestor or many?).

A close look at domestic pigeons – their amazing differences and origins.

How people have chosen traits over time (selection).

Planned versus unplanned selection.

The mystery of where our domestic plants and animals first came from.

What helps humans select traits effectively.

Why Domesticated Life Varies More Than Wild Life

When we look at plants and animals that humans have cultivated or farmed for a long time, we quickly notice something. Individuals of the same domestic variety often look more different from each other than individuals of a single wild species do.

Think about all the different plants and animals humans have raised over thousands of years. They've lived in different climates and under different kinds of care. This makes us think that the main reason they vary so much is because their living conditions weren't as steady or natural as those of their wild ancestors.

There might be another reason, suggested by Andrew Knight: maybe getting extra food contributes to variation.

It also seems that living things need to be exposed to new conditions for several generations before they start showing significant changes. Once an organism starts changing, it often keeps changing for many more generations. We don't know of any case where a changing organism stopped changing while under human care. Our oldest crops, like wheat, still produce new varieties. Our oldest farm animals can still be changed or improved quickly.

How Living Conditions Cause Change

From what I can tell after studying this for a long time, living conditions seem to affect organisms in two main ways:

Directly: Acting on the whole body or just certain parts.

Indirectly: Affecting the reproductive system.

Direct Effects: We need to remember, as Professor Weismann pointed out, that there are always two things involved: the nature of the organism itself, and the nature of the conditions it lives in. The organism's own nature seems more important. Why?

Sometimes, similar changes happen in organisms living under very different conditions.

Other times, different changes happen in organisms living under very similar conditions.

The effects on the offspring can be definite or indefinite.

Definite Effects: Changes are definite when almost all offspring turn out the same way after being exposed to specific conditions for several generations. It's hard to know how much change happens this way. But small changes are likely definite:

Size changing based on the amount of food.

Color changing based on the type of food.

Skin and hair thickness changing based on climate.
Every little difference we see, like in chicken feathers, must have a cause. If that same cause affected many chickens consistently over many generations, they would likely all change in the same way. Think about how a tiny drop of insect poison can cause strange, complex growths (galls) on plants. This shows how big changes could happen in plants just from a chemical change in their sap.

Indefinite Effects: This is when changes are unpredictable and varied. It's a much more common result of changed conditions than definite changes. Indefinite variability has probably been more important in creating our domestic breeds. We see it in the countless small differences between individuals of the same species that can't be explained by inheriting them from parents or ancestors. Even babies in the same litter, or seedlings from the same seed pod, can show clear differences. Rarely, very noticeable changes, almost like deformities ("monstrosities"), appear in one individual out of millions raised in the same place with similar food. There's no clear line between these big changes and smaller variations. All these changes, big or small, that pop up randomly among individuals living together can be seen as the indefinite effects of living conditions. It's like how getting chilled affects different people differently based on their health, causing a cough, a cold, rheumatism, or other issues.

Indirect Effects: Impact on Reproduction

Now let's consider the indirect effects of changed conditions – how they affect the reproductive system. We think variability happens this way partly because the reproductive system is extremely sensitive to any change in conditions.

It also seems related to how new variations appear when different species are crossed (hybridized). Kölreuter and others noted the similarity between variation from crossing and variation from raising organisms under new or unnatural conditions.

Many facts show how easily the reproductive system is affected by even slight changes:

It's easy to tame a wild animal.

It's often very hard to get tamed wild animals to breed successfully in captivity, even if they mate.

Many animals won't breed even if kept in large enclosures in their native country. People often wrongly blame this on messed-up instincts.

Many cultivated plants grow strong and healthy but rarely produce seeds.

Sometimes, a tiny change (like a bit more or less water at a specific time) determines if a plant makes seeds.

I've gathered many details on this, but here's a curious point about breeding animals in captivity:

Meat-eating animals (carnivores), even from tropical regions, often breed fairly well here (except bears, which rarely have young).

Meat-eating birds (carnivorous birds), however, almost never lay fertile eggs in captivity.

Many foreign plants produce pollen that is completely useless, just like in sterile hybrids.

Consider these two points:

Domesticated animals and plants, even if weak or sickly, often breed easily in captivity.

Wild individuals, caught young, tamed, healthy, and long-lived, often fail to reproduce because their reproductive system is somehow seriously affected by subtle causes.

Given this, it shouldn't surprise us that when the reproductive system does work under unnatural conditions (like captivity or cultivation), it might work irregularly. This can lead to offspring being somewhat different from their parents – causing variability.

I should add: some organisms breed freely even under very unnatural conditions (like rabbits and ferrets in small hutches). This shows their reproductive systems aren't easily affected. Similarly, some animals and plants resist changing much under domestication or cultivation. They might vary only slightly, perhaps no more than they would in the wild.

Can Variation Happen Without Sex? Bud Variations

Some scientists used to think all variation was linked to sexual reproduction. This is wrong. I have published a long list of "sporting plants." These are plants that suddenly produce a single bud with a new and different character compared to other buds on the same plant.

These bud variations can be copied by grafting or taking cuttings. Sometimes they even pass on through seeds. They are rare in nature but common under cultivation.

Think about this:

A single bud out of thousands on a tree, living under stable conditions, can suddenly change.

Buds on different trees, under different conditions, sometimes produce nearly the same new variety (like peach trees producing nectarines, or common roses producing moss roses).

This clearly shows that the organism's own nature is more important than the specific conditions in causing a particular variation. The conditions might just be the trigger, like a spark igniting flammable material – the spark doesn't determine the type of fire, the material does.

How Habits Change Organisms Over Time

Changed habits can lead to effects that are passed down (inherited).

Plants: Moving plants to a new climate can change their flowering time, and this change can be inherited.

Animals: Increased use or disuse of body parts has a bigger impact.

Domestic Ducks: I found their wing bones are lighter, and their leg bones are heavier, compared to the whole skeleton, than in wild ducks. This is likely because domestic ducks fly much less and walk more than their wild ancestors.

Cow and Goat Udders: In places where cows and goats are milked regularly, their udders are much larger and more developed than in other places. This is probably another effect of use, and it's inherited.

Drooping Ears: Almost all domestic animals have breeds with drooping ears somewhere in the world. It's been suggested this happens because the ear muscles are used less, as the animals aren't often alarmed. This seems likely.

Linked Changes: Correlated Variation

Many rules govern variation. We only understand a few dimly. Here, I'll just mention correlated variation. This means changing one part of an organism often leads to changes in other parts.

Early Development: Significant changes in an embryo or larva will probably cause changes in the adult animal.

Monstrosities: In animals with major deformities, the connections between very different parts are strange (Isidore Geoffroy St. Hilaire wrote a lot about this).

Breeder Observations: Breeders believe long limbs usually go with a long head.

Odd Correlations: Some links seem random:

Completely white cats with blue eyes are often deaf (though Mr. Tait recently suggested this might only apply to males).

Color is often linked to body constitution. White sheep and pigs can be harmed by certain plants that dark-colored ones eat without problems. Professor Wyman told me about farmers in Virginia whose pigs were all black. They explained that the pigs ate paint-root, which turned their bones pink and made the hooves fall off all pigs except the black ones. So, they specifically chose black piglets to raise.

Hairless dogs tend to have bad teeth.

Animals with long or coarse hair are said to often have long or many horns.

Pigeons with feathers on their feet often have skin between their outer toes.

Pigeons with short beaks have small feet; pigeons with long beaks have large feet.

Conclusion on Correlation: If humans keep selecting for one particular trait and making it stronger, they will almost certainly unintentionally change other parts of the organism too, because of these mysterious laws of correlation.

The Complexity of Variation

The results of all these known and unknown causes of variation are incredibly complex and diverse. It's worth reading books about older cultivated plants like hyacinths, potatoes, or dahlias. It's amazing to see the endless tiny ways varieties and sub-varieties differ in structure and constitution. The whole organism seems to become flexible ("plastic"), changing slightly from its original parent type.

Passing Traits Down: Inheritance

A variation that is not inherited doesn't matter for our discussion. But countless variations are inheritable, from tiny differences to major ones. Dr. Prosper Lucas wrote the best, most detailed books on this.

No breeder doubts that traits tend to be inherited. "Like produces like" is their basic belief. Only theoretical writers have questioned this.

Challenge: If a trait appears often, and we see it in both parent and child, we can't be sure it's inherited. Maybe the same cause affected both of them.

Proof: But imagine a very rare trait appears in a parent (say, one in several million individuals) due to some unusual combination of factors. If that same rare trait then appears in the child, even though they seem to live under the same conditions, the laws of probability strongly suggest it was inherited.

Common Examples: Everyone knows about things like albinism (lack of pigment), unusual skin conditions, or hairy bodies running in families.

If strange and rare traits are inherited, we can easily accept that less strange, more common traits are also inheritable.

Rule of Thumb for Inheritance: Perhaps the best way to think about it is: inheritance of every trait is the rule, and non-inheritance is the exception.

What We Don't Know About Inheritance

The specific rules of inheritance are mostly unknown. Nobody can explain:

Why the same trait is inherited in some individuals or species but not others.

Why children often resemble grandparents or even more distant ancestors for certain traits (reversion).

Why a trait is often passed from one parent to both sexes of offspring, or only to one sex (usually the same sex as the parent, but not always).

Important Patterns in Inheritance:

Traits appearing in males of domestic breeds are often passed mainly or only to male offspring.

A key rule: Whatever age a trait first appears in a parent, it tends to appear in the offspring at about the same age (sometimes earlier). This makes sense for things like cattle horns (appear when mature) or silkworm traits (appear at the matching caterpillar or cocoon stage). But inherited diseases and other facts suggest this rule applies more broadly. Even when there's no obvious reason for a trait to appear at a specific age, it tends to follow the parent's timing. I believe this rule is very important for understanding how embryos develop.

(Note: This timing refers to when the trait shows up, not the original cause. For example, a calf might inherit the potential for long horns from its long-horned father, but the horns only grow long later in life. The trait's appearance is late, but the cause was present from conception.)

Going Backwards: Reversion to Wild Ancestors

Let's talk about reversion. Some scientists claim that when domestic varieties escape and live wild, they gradually but always change back to look like their original wild ancestors. This has been used to argue that we can't learn about wild species by studying domestic breeds.

I've tried hard to find solid facts supporting this claim, but haven't found them. It would be very difficult to prove:

Many highly developed domestic breeds probably couldn't survive in the wild anyway.

In many cases, we don't even know what the original wild ancestor looked like, so we couldn't tell if the animals had reverted perfectly.

To test this properly, only a single variety could be released into a new area to prevent interbreeding with other varieties confusing the results.

However, since domestic varieties do sometimes show traits from their ancestors, it seems possible that if we could get breeds (like different cabbages) to live wild or grow them in very poor soil for many generations, they might largely or completely revert to the wild ancestor. (Poor soil itself would also have a direct effect).

Whether this experiment would work isn't crucial for my argument. The experiment itself involves changing the conditions of life. The important point is this: Is there any evidence that domestic varieties tend to lose their special traits and revert while being kept under the same conditions and in large enough groups where interbreeding would mix traits? No, there isn't. It goes against all experience to say we couldn't keep breeding our distinct cart horses, racehorses, cattle, poultry, and vegetables for countless generations.

How Domestic Breeds Compare to Wild Species

When we compare domestic breeds (races or varieties) to closely related wild species:

Domestic breeds usually show less uniformity among individuals than wild species do.

Domestic breeds often have a somewhat "monstrous" quality. This means that while they differ from each other and from wild relatives in small ways, they often differ extremely in one specific part.

Aside from these points (and the fact that varieties are fertile when crossed, which we'll discuss later), domestic breeds differ from each other much like closely related wild species differ – just usually less dramatically.

This similarity must be true because experts often disagree on whether certain domestic forms are distinct species or just varieties. If there were a clear, sharp line between a domestic breed and a wild species, this confusion wouldn't happen so often.

Some say domestic breeds don't differ enough to be considered different genera. This isn't quite right, but scientists disagree a lot on what makes characters "generic" anyway – it's subjective right now. When we understand how genera form in nature, we'll see why we shouldn't expect huge (generic level) differences between domestic breeds often.

One Ancestor or Many? The Origin Puzzle

Trying to figure out how much difference exists between domestic breeds gets tricky because we often don't know if they came from one wild ancestor species or several.

Knowing this would be interesting. For example, if we could prove that greyhounds, bloodhounds, terriers, spaniels, and bulldogs all came from a single wild species, it would make us seriously question whether closely related wild species (like the many types of foxes around the world) are truly fixed and unchanging.

Personally, I don't think all the differences between dog breeds arose after domestication. I suspect some difference is because they descended from different wild species.

However, for some other domestic animals with very distinct breeds, there's good reason to believe they all came from a single wild ancestor.

Did Humans Pick Animals That Were Already Prone to Vary?

Some people assume humans chose animals and plants for domestication that naturally tended to vary a lot and could survive in different climates.

I agree these abilities make domestic species more valuable. But how could an early human, first taming an animal, possibly know if its descendants would vary or survive elsewhere?

Did the fact that donkeys and geese vary little stop us from domesticating them?

Did the reindeer's trouble with heat, or the camel's trouble with cold, prevent their domestication?

I believe that if we took any group of wild animals and plants (equal in number and diversity to our current domestic ones) and managed to breed them under domestication for the same number of generations, they would, on average, vary just as much as the ancestors of our current domestic species did.

Finding the Roots of Ancient Domestics

For most animals and plants domesticated long ago, it's impossible to say for sure if they came from one or several wild species.

Argument for Multiple Origins: People who believe in multiple origins point to ancient Egyptian monuments and remains from Swiss lake dwellings. These show diverse breeds existed very early on, some looking just like modern breeds.

Counter-Argument: This evidence mainly shows that civilization is older than we thought, and animals were domesticated much earlier. The Swiss lake dwellers grew various crops (wheat, barley, peas, poppies, flax) and had several domestic animals. They traded with others. This implies they were already quite civilized, which means there must have been a long period before that when less advanced people in different areas could have kept animals that varied and developed into distinct races. Since geologists found stone tools proving humans existed extremely long ago, and even today almost every tribe has domesticated dogs, there was plenty of time for this.

Likely Origins of Specific Animals (My View)

The exact origin of most domestic animals will probably always be unclear. But based on my research:

Dogs: After studying dogs worldwide, I concluded that several wild dog-like species (Canidae) were tamed. Their bloodlines, sometimes mixed, run in our modern breeds.

Sheep and Goats: I can't form a definite opinion.

Cattle: Based on information from Mr. Blyth about Indian humped cattle (zebu) – their habits, voice, constitution, and structure – it's almost certain they came from a different wild ancestor than European cattle. Some experts (like Professor Rütimeyer, through excellent research) believe European cattle might have had two or three wild ancestors (whether we call them species or not).

Horses: For reasons I can't detail here, I lean towards believing all horse races belong to the same species, though several authors disagree.

Chickens: Having kept, bred, crossed, and examined skeletons of nearly all English chicken breeds, it seems almost certain they all descend from the wild Indian junglefowl (Gallus bankiva). Mr. Blyth and others who studied this bird in India agree.

Ducks and Rabbits: Although some breeds look very different, the evidence is clear they all descend from the common wild duck and the wild rabbit.

Problems with Believing Every Breed Had a Wild Twin

Some writers take the idea of multiple origins too far. They think every single breed that reproduces consistently ("breeds true"), no matter how small the difference, must have had its own separate wild ancestor species.

Implausibility: This would mean there were dozens of wild cattle species, dozens of wild sheep species, and several wild goat species just in Europe! One writer even suggested Britain alone had eleven unique wild sheep species!

Contradiction: Britain has no unique native mammals now, and France has few different from Germany's. Yet these countries have many unique breeds of cattle, sheep, etc. If these breeds didn't originate in Europe through variation and selection, where did they come from? The same applies to India.

Dog Example: Even with dogs, which I agree likely came from several wild species, there has clearly been a huge amount of inherited variation since domestication. Who believes animals exactly like Italian Greyhounds, Bloodhounds, Bulldogs, Pugs, or Blenheim Spaniels – so different from any wild dog – ever existed in the wild?

Crossing Isn't Enough: People often casually say dog breeds came from crossing a few wild species. But crossing only creates offspring that are somewhat intermediate between the parents. To explain today's diverse breeds by crossing alone, we'd have to assume extremely different wild forms (like greyhounds and bulldogs) already existed. Also, the power of crossing to create distinct, stable races has been overstated. Occasional crosses can help modify a race if combined with careful selection, but creating a stable intermediate race between two very different pure breeds is very hard. Sir J. Sebright tried this specifically and failed. The first cross between two pure breeds often looks uniform, but breeding those mixed offspring (mongrels) together for generations results in wide variation, showing how difficult the task is.

Focusing on Pigeons: A Case Study

Believing it's best to study a specific group in detail, I chose domestic pigeons.

I've kept every breed I could get.

I received skins from around the world (thanks to Hon. W. Elliot from India and Hon. C. Murray from Persia).

I read many old and important books on pigeons in different languages.

I spent time with top pigeon breeders ("fanciers") and joined London Pigeon Clubs.

The Amazing Variety of Pigeons

The differences between pigeon breeds are truly astonishing.

English Carrier vs. Short-faced Tumbler: Compare their beaks – wildly different, leading to different skull shapes. The Carrier (especially males) also has large fleshy growths (caruncles) around the head, long eyelids, big nostril openings, and a wide mouth gape. The Short-faced Tumbler has a tiny beak, almost like a finch's.

Common Tumbler: Has the strange, inherited habit of flying high in a group and tumbling head over heels.

Runt: A huge bird with a long, heavy beak and large feet. Some Runt sub-breeds have very long necks, wings, or tails; others have very short tails.

Barb: Related to the Carrier, but has a very short, wide beak instead of a long one.

Pouter: Has a very long body, wings, and legs. Its huge crop, which it proudly inflates, is amazing and almost comical.

Turbit: Has a short, cone-shaped beak and a line of reversed feathers down its chest. It constantly puffs out the top of its throat slightly.

Jacobin: Has neck feathers reversed so much they form a hood. Its wing and tail feathers are relatively long.

Trumpeter and Laugher: Make cooing sounds very different from other breeds.

Fantail: Has 30 or even 40 tail feathers instead of the normal 12-14 found in all other pigeons. It holds these feathers spread out and so upright that the head and tail touch in good birds. Its oil gland is missing.

Others: Several other less distinct breeds exist.

Differences Inside and Out

Skeletons: Bone development in the face (length, width, curve) varies enormously. The lower jaw shape varies remarkably. The number of tail and lower back vertebrae varies, as does the number of ribs and their shape. Holes in the breastbone vary in size and shape. The wishbone (furcula) arms vary in how far apart they are and their size.

Other Traits: Many other features vary:

Width of the mouth opening.

Length of eyelids and nostrils.

Tongue length (not always matching beak length).

Size of the crop and upper throat.

Presence or absence of the oil gland.

Number of main wing and tail feathers.

Relative length of wing, tail, leg, and foot.

Number of scales on the toes.

Amount of skin between the toes.

Timing of getting adult feathers.

Type of down on hatchlings.

Egg shape and size.

Way of flying.

Voice and personality (in some breeds).

Slight differences between males and females in certain breeds.

If They Were Wild Birds...

You could easily pick at least 20 pigeon breeds that, if shown to a bird expert (ornithologist) who was told they were wild, would certainly be classified as distinct species.

Furthermore, I doubt any ornithologist would put the English Carrier, Short-faced Tumbler, Runt, Barb, Pouter, and Fantail in the same genus (a higher classification level). Especially since each of these main breeds has several sub-breeds that breed true – which the expert would likely call more distinct species.

Why All Pigeons Likely Came From One Ancestor: The Rock Pigeon

Despite these huge differences, I am convinced that the common scientific opinion is correct: all domestic pigeon breeds descended from the wild Rock Pigeon (Columba livia). (This includes several wild geographic races or sub-species of C. livia that differ only slightly).

Here are my reasons (some apply to other domestic animals too):

Multiple Ancestors Unlikely: If the breeds are not just varieties of the Rock Pigeon, they must have come from at least 7 or 8 different wild ancestor species. You couldn't create the current breeds by crossing fewer types. (For example, how could you get a Pouter with its huge crop unless one of the original ancestors already had one?). These supposed wild ancestors must have all been rock-dwelling pigeons (like domestic breeds, they don't like perching or breeding in trees). But besides C. livia and its sub-species, only 2 or 3 other species of true rock pigeons are known, and they don't look like any domestic breeds.

So, these imagined ancestors must either:

Still exist somewhere, unknown to scientists (unlikely, given their size, habits, and strange features).

Have gone extinct in the wild. But cliff-nesting birds that are good flyers are hard to exterminate. The common Rock Pigeon hasn't been wiped out even on small British islands or Mediterranean coasts. So, assuming many similar species were exterminated seems very unlikely.

No Feral Breeds: Domestic pigeon breeds have been taken all over the world. Some must have been brought back to their supposed "native" regions. Yet, none of the distinct breeds have successfully established wild (feral) populations. However, the dovecote pigeon (which is just a slightly changed Rock Pigeon) has become feral in several places.

Domestication is Hard: We know it's difficult to get wild animals to breed reliably in captivity. Yet, the multiple-origin idea requires assuming that ancient, "half-civilized" humans managed to thoroughly domesticate at least 7 or 8 different pigeon species so well that they all became very fertile under human care.

Unique Strange Traits: The domestic breeds generally match the wild Rock Pigeon (in body type, habits, voice, color, etc.), but they have extremely unusual ("abnormal") features in certain parts (like the Carrier's beak, Jacobin's hood, Pouter's crop, Fantail's tail) that are not seen anywhere else in the entire pigeon family (Columbidae). The multiple-origin idea requires us to believe not only that ancient humans domesticated several species, but that they specifically picked out incredibly strange, abnormal wild species, and that all these unique wild species later went extinct or became unknown. This chain of events is extremely improbable.

Color Patterns Reappear: The wild Rock Pigeon has a specific color pattern: slate-blue body, white lower back (or bluish in the Indian sub-species), dark band at the tail tip, outer tail feathers edged white at the base, and two black bars on the wings. Some wild or semi-domestic ones also have black checkered patterns on the wings. No other species in the entire pigeon family has all these marks together.

Observation: In every domestic breed, even the most carefully bred ones, all these markings sometimes appear perfectly, down to the white edging on the tail feathers.

Crossing Experiment: When you cross birds from two or more distinct breeds that are not blue and don't have these marks, their mixed offspring (mongrels) often suddenly develop these Rock Pigeon markings.

Example: I crossed pure-breeding white Fantails with black Barbs (blue Barbs are extremely rare). The offspring were black, brown, or mixed colors. I also crossed a Barb with a Spot (a white bird with a red tail and forehead spot, breeds very true). The offspring were dark and mixed. Then, I crossed one of the Barb-Fantail mongrels with one of the Barb-Spot mongrels. They produced a bird that was beautifully blue, with the white lower back, double black wing bars, and barred, white-edged tail – just like a wild Rock Pigeon!

Explanation (Single Origin): This makes perfect sense if all breeds descended from the Rock Pigeon. It's reversion – going back to ancestral characteristics.

Alternative Explanations (Multiple Origins - Highly Improbable): If we deny the single origin, we have two unlikely options:

Option 1: All the imagined 7-8 original wild ancestor species just happened to be colored and marked exactly like the Rock Pigeon (even though no other existing species is). This way, each breed would just have a tendency to revert to those same markings.

Option 2: Every breed, even the purest, must have been crossed with a Rock Pigeon sometime within the last dozen (or maybe 20) generations. Why so recent? Because we know that the tendency to revert to traits from a cross fades with each generation as the "foreign blood" gets diluted. Reversion to traits from distant ancestors (more than ~20 generations back) through crossing isn't really seen. (Note: This is different from reversion within a pure line to a trait lost generations ago. That tendency can seemingly last forever).

Crossed Breeds Are Fertile: The hybrids or mongrels produced by crossing any of the domestic pigeon breeds are perfectly fertile. I know this from my own experiments with the most distinct breeds. However, it's extremely rare for hybrids from two genuinely distinct wild animal species to be perfectly fertile.

Some argue that long domestication might make species less prone to producing sterile offspring when crossed. This might be true for closely related species (like perhaps dog breeds originating from different, but similar, wild ancestors).

But assuming domestication could make species as different as Carriers, Tumblers, Pouters, and Fantails (if they were originally distinct species) produce perfectly fertile offspring together seems like a huge stretch.

Conclusion: Rock Pigeon is the Ancestor

Putting all these reasons together:

The improbability of humans domesticating 7-8 unknown, rock-dwelling wild species long ago.

These supposed species being unknown now and not becoming feral.

These species having strange traits unlike any other pigeons, yet otherwise resembling the Rock Pigeon.

The reappearance of Rock Pigeon colors/marks in all breeds (pure or crossed).

The perfect fertility of mongrels between all breeds.

...we can safely conclude that all our domestic pigeon breeds descended from the Rock Pigeon (Columba livia) and its geographical sub-species.

More Evidence for Rock Pigeon Origin:

Domestication: Wild C. livia can be domesticated (in Europe and India) and matches the domestic breeds in habits and many structural points.

Gradual Links: Although extreme breeds (Carrier, Tumbler) look very different from the Rock Pigeon, by comparing various sub-breeds (especially from different regions), we can often find an almost continuous series of forms linking them back to the Rock Pigeon. (This isn't possible for all breeds, but for many).

Variable Traits: The main features that define each breed (like the Carrier's wattle and beak length, the Tumbler's short beak, the Fantail's tail feathers) are also the features that vary the most within that breed. (We'll see why when we discuss Selection).

Long History of Care: Pigeons have been watched, cared for, and loved by many people for thousands of years. Records go back to the 5th Egyptian Dynasty (around 3000 BC). Romans paid huge prices for them and tracked their pedigrees. Akbar Khan in India (around 1600 AD) kept over 20,000 pigeons and received rare birds from other rulers; he even improved breeds by crossing them ("which method was never practised before"). The Dutch were also keen pigeon keepers around that time. This long history and intense human interest are crucial for understanding how much variation occurred (again, explained by Selection).

Why Pigeons Changed So Much (Preview):

This long history of human care is key.

We'll see later how Selection explains the huge amount of change.

Selection also helps explain why breeds often have somewhat "monstrous" or exaggerated features.

It was also helpful that pigeons can be easily paired for life, allowing different breeds to be kept pure even in the same enclosure (aviary). This made creating new breeds much easier.

A Lesson from Breeders

I've spent time discussing pigeon origins because when I first started keeping them, knowing they bred true, I found it just as hard to believe they all came from one ancestor as any scientist finds it hard to believe wild finch species evolved from a common ancestor.

One thing struck me: Almost all breeders I've talked to or read about (whether they raise animals or plants) are convinced that the different breeds they specialize in came from separate, distinct wild species.

Ask a Hereford cattle breeder if his cows could have come from Longhorns, or if both came from one ancestor, and he'll laugh at you.

I never met a pigeon, chicken, duck, or rabbit breeder who didn't believe each main breed was a distinct original species.

Van Mons, writing about pears and apples, clearly didn't believe different varieties (like a Ribston-pippin and a Codlin-apple) could have come from seeds of the same tree.

Why do breeders think this? I think it's simple: They work closely with the breeds for so long that they become deeply impressed by the differences between them. They know each breed varies slightly (that's how they win prizes – by selecting slight improvements), but they ignore the bigger picture. They refuse to mentally add up all those small differences accumulating over many generations.

A Cautionary Note: Maybe naturalists (scientists) can learn from this. Naturalists often know less about inheritance laws than breeders do, and no more about the intermediate steps in long lines of descent. Yet, many naturalists do accept that domestic breeds came from common parents. Perhaps these same naturalists should be more cautious when they dismiss the idea that wild species might also be descendants of other species.

How Did Humans Create Domestic Breeds?

Let's briefly look at the steps involved in producing domestic races, whether from one or several wild ancestors.

Some change comes from the direct effect of living conditions.

Some comes from habit (use and disuse).

But it would be bold to claim these factors alone could explain the huge differences between a heavy dray horse and a sleek racehorse, a greyhound and a bloodhound, or a Carrier pigeon and a Tumbler pigeon.

A key feature of domestic breeds is that they are adapted to human needs or preferences, not necessarily to their own survival or well-being.

Some useful variations might have appeared suddenly:

Many botanists think the Fuller's Teasel (with hooks used for treating cloth, better than any machine) is just a variety of the wild teasel (Dipsacus), and this change might have happened in one step in a seedling.

The short-legged Turnspit Dog might have appeared suddenly.

The Ancon sheep (with short legs) is known to have arisen suddenly.

But when we look at the vast range of breeds:

Dray horse vs. Racehorse

Dromedary vs. Bactrian Camel

Sheep adapted for flat pastures vs. mountain pastures

Sheep with wool good for different purposes

Many dog breeds, each useful to humans in different ways

Game-cocks (fighters) vs. calm breeds

"Everlasting layer" chickens (don't want to sit on eggs) vs. tiny Bantam chickens

Countless plant varieties for farming, cooking, orchards, and gardens – useful at different times or for different needs, or just beautiful to us.

...we must look beyond simple variability. We can't assume all these breeds suddenly appeared as perfect and useful as they are now. We often know this wasn't the case.

The Key: Human Selection Power

The answer is human power of accumulative selection.

Nature provides variations generation after generation.

Humans notice these variations and add them up over time, guiding the changes in directions useful to them.

In this sense, humans can be said to have made useful breeds for themselves.

Selection Isn't Just a Theory – It Works

The power of selection is real.

We know several skilled breeders have significantly changed their cattle and sheep breeds within their own lifetimes. Reading books about breeding and seeing the animals helps grasp this.

Breeders often talk about an animal's body as being "plastic" – something they can mold almost as they wish.

Youatt (an expert on farming and animals) called selection "the magician's wand," allowing farmers to create almost any form they please.

Lord Somerville said breeders seemed to draw a perfect shape on a wall and then bring it to life in their sheep.

In Saxony (Germany), selecting Merino sheep is a profession. Experts study sheep on tables like art critics study paintings, marking and classing them multiple times to pick the very best for breeding.

Proof of Success:

The huge prices paid for animals with good pedigrees.

These top animals are exported all over the world.

Improvement is usually not due to crossing different breeds. Best breeders avoid this, except maybe between very closely related sub-breeds.

When a cross is made, careful selection becomes even more critical.

The Real Power of Selection:

It's not just about picking out an obviously different variety and breeding from it. That's too simple.

The real importance is the huge effect of accumulating tiny differences – differences an untrained eye wouldn't even notice – in one direction over many generations. (I've tried to see these tiny differences myself and failed).

Becoming a top breeder requires rare skill: accuracy of eye and judgment. Not one person in a thousand has it.

Even with talent, it takes years of study and a lifetime of persistent effort to make great improvements. Without these qualities, failure is certain. Few realize the natural skill and practice needed just to be a good pigeon fancier.

Selection in Gardening (Horticulture)

Gardeners use the same principles, but the starting variations are often more sudden or distinct ("abrupt").

Nobody thinks our best flowers or fruits came from a single lucky variation from the wild ancestor. Records show this isn't true (e.g., the common gooseberry gradually got bigger over time).

We see amazing improvements in flowers by comparing today's versions to drawings from just 20-30 years ago.

Once a plant variety is fairly well established, seed growers don't pick the single best plant. Instead, they walk through their fields and pull out the "rogues" – plants that don't meet the standard for that variety.

This "roguing" is essentially what animal breeders do too – almost everyone avoids breeding from their worst animals.

Selection Focuses on Specific Parts

We can see the effects of focused selection by comparing different parts of plant varieties:

Flower Garden: Look at different cabbage varieties – their leaves are incredibly different, but their flowers look almost identical. Now look at pansies (heartsease) – their flowers are very different, but their leaves are quite similar.

Kitchen Garden: Compare different types of gooseberries – the fruits vary hugely in size, color, shape, and hairiness, but their flowers show only slight differences.

This doesn't mean varieties that differ a lot in one part don't differ at all in others. Because of correlated variation (where changing one part affects others), there will likely be some differences elsewhere. But, as a general rule:

Continuously selecting small variations in leaves leads to races differing mainly in their leaves.

Continuously selecting variations in flowers leads to races differing mainly in their flowers.

Continuously selecting variations in fruit leads to races differing mainly in their fruit.

Selection Isn't New – It's Ancient

Someone might object that methodical, planned selection has only been practiced for about 75 years. It's true that it's received more attention recently, with many books published, leading to faster results.

But it's completely wrong to think the principle of selection is new.

I could cite ancient texts showing the principle was understood long ago.

Even in rough periods of English history, choice animals were imported, laws banned exporting them, and undersized horses were ordered destroyed (similar to gardeners "roguing" plants).

I found selection clearly described in an ancient Chinese encyclopedia.

Classical Roman writers laid down specific rules.

Passages in Genesis show people paid attention to the color of domestic animals very early on.

Pliny wrote that ancient "savages" sometimes crossed their dogs with wild relatives to improve them (and modern ones still do).

Savages in South Africa match their draft cattle teams by color. Some Eskimos match their dog teams.

Livingstone noted that people in interior Africa who hadn't met Europeans highly valued good domestic breeds.

These examples don't all show active selection for change, but they prove that people have paid careful attention to breeding animals since ancient times, and even the "lowest savages" do now. It would be strange if they didn't, because the inheritance of good and bad qualities is so obvious.

Methodical Selection vs. Unconscious Selection

Methodical Selection: Today, top breeders deliberately try to create a new strain or sub-breed that's better than anything else, with a clear goal in mind.

Unconscious Selection: For our purpose, another type of selection is even more important. This happens when everyone simply tries to own and breed from the best individuals they can find, without any specific plan or expectation of permanently changing the breed.

Example: Someone wanting Pointer dogs tries to get good ones and breeds from their best dogs. They don't intend to alter the Pointer breed itself.

Effect: We can infer that this process, carried on for centuries, would gradually improve and modify any breed. It's the same process used by famous breeders like Bakewell and Collins, just less planned and slower. They greatly modified their cattle breeds even within their lifetimes using methodical selection.

Detecting Change: These slow, gradual ("insensible") changes are impossible to notice unless someone made accurate measurements or drawings of the breed long ago for comparison.

Evidence: Sometimes, unchanged or less-changed individuals of the same breed still exist in less developed areas where less improvement has occurred. There's reason to think the King Charles's Spaniel has been significantly modified unconsciously since King Charles's time. Experts believe the Setter dog was slowly derived from the Spaniel. The English Pointer has changed greatly in the last century (partly due to deliberate crosses with Foxhounds, but the overall change was gradual and unconscious). It's now so different that although the original Spanish Pointer came from Spain, Mr. Borrow (an expert on Spain) told me he hasn't seen any native Spanish dogs that look like our modern English Pointer.

More Examples of Unconscious Selection

Racehorses: Through this slow selection (and training), English racehorses became faster and larger than their Arab ancestors. Arabs now get a weight advantage in some races.

Cattle: Lord Spencer and others showed how English cattle grew heavier and matured faster compared to older stock.

Pigeons: By comparing old descriptions of Carrier and Tumbler pigeons (in Britain, India, Persia) with modern ones, we can see the gradual stages they passed through, becoming so different from the wild Rock Pigeon.

Youatt's Example: Unintentional Creation of Two Breeds
Youatt gave a great example of unconscious selection producing an unexpected result: two distinct strains. Two flocks of Leicester sheep, kept by Mr. Buckley and Mr. Burgess, were bred purely from Mr. Bakewell's original stock for over 50 years. Everyone agreed no outside blood was introduced. Yet, the sheep in the two flocks became so different they looked like separate varieties. The breeders never intended or expected this outcome.

Selection Even Without Conscious Thought

Even if some "savage" tribes never think about inherited traits, they would naturally try to save their most useful animals during hard times (famines, etc.). These chosen animals would survive and leave more offspring than inferior ones. So, a kind of unconscious selection would still happen. The people of Tierra del Fuego value their dogs so much that during famine, they would kill and eat their old women before their dogs.

Unconscious Selection in Plants

The same gradual improvement happens in plants. We see it by comparing modern garden flowers (like pansies, roses, pelargoniums, dahlias) to older varieties or their wild ancestors. The increased size and beauty came from gardeners occasionally saving the best individuals over time.

Nobody expects to get a top-quality pansy or dahlia by planting seed from a wild plant.

Nobody expects to get a delicious melting pear from wild pear seed (though you might get lucky with a wild-growing seedling if it came from garden stock).

Pliny's description suggests pears in Roman times were poor quality.

People are often surprised at gardeners' skill in producing amazing results from poor starting material. But the "art" was simple and largely unconscious regarding the final outcome. It involved:

Always growing the best variety known at the time.

Sowing its seeds.

If a slightly better variety appeared by chance, selecting it.

Repeating the process.

The Roman gardeners who grew the best pears they could find never dreamed of the amazing pears we eat today. But we owe our excellent fruit partly to them simply choosing and saving the best they had.

Why We Don't Know the Wild Ancestors of Many Plants

This large amount of change, slowly and unconsciously accumulated over centuries or millennia, explains why we often can't identify the wild parent species of plants long cultivated in our gardens. If it took that long to improve them to their current usefulness, it makes sense why places inhabited by non-agricultural peoples (like Australia or the Cape of Good Hope) haven't given us any native plants worth widespread cultivation. It's not that these places lack potentially useful plants; it's that their native plants haven't been improved through generations of selection like those in anciently civilized countries.

A Note on "Natural Selection" in Domestic Animals

Animals kept by non-industrialized people often have to find their own food, at least sometimes. In two different environments, individuals of the same species with slightly different body types or constitutions might do better in one place than the other. This could lead to two sub-breeds forming through a process of "natural selection" (which will be explained more later). This might partly explain why varieties kept by "savages" sometimes seem more distinct, like true species, compared to breeds in civilized countries.

Why Domestic Breeds Suit Human Needs

The idea that human selection is the main driver explains why domestic breeds have structures or habits adapted to human wants or fancies.

We can also understand:

Why breeds often have unusual or "abnormal" features: Humans notice anything new or strange. The more unusual a trait was when it first appeared, the more likely it was to catch someone's eye.

Why external parts differ more than internal organs: Humans can mostly only select for traits they can see. They rarely care about internal differences.

Humans select, Nature provides: Humans can only work with variations that nature first provides, usually in small degrees.

Nobody would try to "make" a Fantail pigeon unless they first saw a pigeon with a slightly unusual tail.

Nobody would try to "make" a Pouter unless they saw a pigeon with a slightly enlarged crop.

Saying someone "tried to make" a Fantail is probably wrong in most cases. The person who first selected a pigeon with a slightly fuller tail likely never imagined what its descendants would become after long periods of partly unconscious and partly planned selection.

Maybe the ancestor of all Fantails had only 14 tail feathers that were slightly spread out (like the modern Java Fantail, or like occasional pigeons in other breeds that can have up to 17 tail feathers).

Maybe the first Pouter didn't inflate its crop much more than the modern Turbit puffs its throat (a habit ignored by breeders because it's not a desired trait).

Don't Underestimate Small Differences

You don't need a huge change to catch a breeder's eye. Fanciers notice extremely small differences. It's human nature to value any novelty, however slight, if it's in something you own.

Also, don't judge how much value people used to place on slight differences by today's standards. Now that many distinct breeds are established, small variations that pop up are often rejected as "faults" or deviations from the breed standard.

Example: The common goose hasn't produced many distinct varieties. So, the Toulouse goose and the common goose, which differ mainly in color (a very changeable trait), have recently been shown as separate "breeds" at poultry shows.

Why Breed Origins Are Often Mysterious

These ideas help explain why we know so little about the origin or history of most domestic breeds.

A breed doesn't usually have a distinct starting point, much like a dialect of a language.

Someone saves and breeds from an individual with a slight difference, or carefully pairs their best animals. The improved animals slowly spread locally.

At this stage, they probably don't have a name, and because they aren't highly valued yet, nobody records their history.

As they are improved further by the same slow process, they spread more widely, become recognized as distinct and valuable, and probably get a local name.

In places with little travel or communication, a new sub-breed would spread slowly.

Once the desirable traits ("points of value") are recognized, the principle of unconscious selection will always tend to slowly enhance those features (though maybe more at some times than others, depending on fashion or local conditions).

The chance that anyone kept records of these slow, variable, gradual changes is extremely small.

What Helps Humans Select Effectively?

What circumstances make human selection more powerful or less powerful?

Lots of Variability: This is obviously helpful. It provides the raw material for selection to work on. However, even just normal individual differences are usually enough to allow significant change in almost any desired direction, if selection is done very carefully.

Large Numbers of Individuals: This is extremely important. Variations useful or pleasing to humans only appear occasionally. Keeping a large number of individuals greatly increases the chance that such variations will appear.

Marshall noted that sheep in parts of Yorkshire couldn't be improved because they belonged to poor people in small flocks.

Nurserymen, who keep large stocks of plants, are usually much better at finding new, valuable varieties than amateur gardeners.

You need favorable conditions to raise large numbers. If individuals are scarce, people will breed from all of them, good or bad, which prevents selection.

High Value Placed on the Organism: This might be the most important factor. The animal or plant needs to be valued enough by humans that they pay close attention even to the slightest differences in its qualities or structure. Without this close attention, selection can't happen.

Someone once seriously remarked how fortunate it was that the strawberry started varying just when gardeners started paying attention to it! The truth is, strawberries likely always varied since being cultivated, but people ignored the small variations. As soon as gardeners started picking plants with slightly larger, earlier, or better fruit, raising seedlings from them, and then picking the best seedlings to breed from again – that's when (with some help from crossing different strawberry species) the many excellent modern varieties appeared.

Ability to Prevent Unwanted Crossing: For animals, being able to control mating is important for creating new races, especially in areas where other breeds already exist. Fences enclosing land help with this. Nomadic peoples or those living on open plains rarely have more than one breed of the same animal.

Pigeons mating for life is very convenient for breeders. It allows many different breeds to be kept pure and improved even when housed together in the same aviary. This must have greatly helped the formation of new pigeon breeds.

Pigeons also reproduce quickly and in large numbers. Inferior birds can be easily removed (and used for food).

Difficulty Preventing Crossing: This hinders selection. Cats, because they roam at night, are hard to pair intentionally. Even though cats are popular pets, we rarely see distinct breeds maintained for long. The breeds we do see are almost always imported.

Lack of Selection Pressure: While some animals might naturally vary less than others, the rarity or absence of distinct breeds in cats, donkeys, peacocks, and geese might be largely because selection hasn't been applied effectively:

Cats: Difficulty in controlled pairing.

Donkeys: Usually kept in small numbers by poor people who pay little attention to breeding (though recently, careful selection in Spain and the US has surprisingly improved donkeys).

Peacocks: Not easily raised in large numbers.

Geese: Valued mainly for only two things (food and feathers), and people haven't taken much pleasure in showing off distinct breeds. The goose itself also seems relatively resistant to change under domestication, although it has varied slightly.

Is There a Limit to Variation?

Some writers claim that domestic animals and plants reach a limit of variation, beyond which they cannot change further.

It seems rash to claim the limit has been reached in any specific case. Almost all our domestic animals and plants have been greatly improved recently, which implies ongoing variation.

It also seems rash to claim that traits currently at their maximum development couldn't start varying again under new conditions after staying fixed for centuries.

No doubt, as Mr. Wallace rightly noted, a limit will eventually be reached. For example, there must be a physical limit to how fast any land animal can run, based on friction, body weight, and muscle power.

Domestic Varieties Differ More Than Wild Species
But what matters for our discussion is this: Domestic varieties of the same species often differ from each other more in the characters that humans have selected for, than distinct wild species of the same genus differ from each other.

Size: Isidore Geoffroy St. Hilaire proved this for size.

Color & Hair: It's likely true for color and hair length too.

Speed & Strength: Regarding speed (which depends on many body features), the racehorse Eclipse was far faster than any wild horse relative. A dray horse is incomparably stronger than any two related wild species.

Plants: Seeds of different bean or corn varieties probably differ more in size than seeds of different wild species within those plant families. The same is true for the fruit of different plum varieties, and even more so for melons and many other cases.

Summary: Where Do Domestic Breeds Come From?

To sum up how our domestic animal and plant breeds originated:

Changed Living Conditions: Very important for causing variability, both directly (affecting the body) and indirectly (affecting reproduction).

Variability Itself: Probably not something that happens automatically under all circumstances. Whether variations last depends on the balance between Inheritance (passing traits on) and Reversion (tendency to go back to older forms).

Laws of Variation: Variability is controlled by many unknown laws. Correlated Growth (changing one part affects others) is likely the most important one we glimpse.

Direct Effect of Conditions: Conditions themselves cause some definite changes, but we don't know how much.

Use and Disuse: Habit (using or not using parts) may have some effect, perhaps a large one.

Complexity: All these factors make the final result incredibly complex.

Crossing Original Species: In some cases, crossing originally distinct wild species seems to have played a role.

Crossing Existing Breeds: Once several breeds exist, occasionally crossing them, combined with selection, has surely helped create new sub-breeds. But the importance of crossing has often been exaggerated, especially for animals and seed-grown plants. (For plants copied by cuttings, buds, etc., crossing is immensely important because breeders can ignore the resulting variability and sterility, but these plants are temporary and less relevant to our long-term view).

The Dominant Power: Selection: Over all these other causes of change, the accumulative action of Selection seems to have been the most powerful force. This includes selection applied methodically and quickly by skilled breeders, and selection applied unconsciously and slowly but perhaps even more effectively by many people over long periods.

Variation and Differences in the Wild

Before we use the ideas from the last chapter (about domesticated plants and animals) to understand nature, we need to ask: do wild plants and animals actually change or vary?

To really cover this well, I should provide a long list of facts, but I'll save that for a future book. I also won't get into arguments about how exactly to define the word "species." No single definition satisfies all scientists (naturalists). However, every naturalist basically knows what they mean by it. Often, the term includes the idea of a separate, unknown act of creation in the past.

The word "variety" is almost as hard to define. Usually, it implies that the individuals came from the same ancestor, although this is rarely provable.

We also have things called "monstrosities." These are significant changes in structure, usually harmful or not useful, that gradually blend into what we call varieties.

Some scientists use "variation" technically to mean changes caused directly by living conditions (like climate or food), and they assume these changes aren't inherited. But can we be sure?

Think about seashells that are smaller in less salty water (like the Baltic Sea).

Or plants that are stunted on high mountain tops.

Or animals from cold regions having thicker fur.
Couldn't these traits sometimes be inherited, at least for a few generations? If they were inherited, we'd probably call that form a variety.

Do Big, Sudden Changes Happen and Stick Around in Nature?

We sometimes see large, sudden changes in domesticated plants and animals. But do these kinds of changes ever become permanent features of a wild species? It seems unlikely.

Every part of a living thing is beautifully connected to its complex environment and way of life. It feels as improbable that any part could suddenly appear fully formed and perfect as it is that a complex machine could be invented perfectly in one go.

Occasionally, domesticated animals are born with strange features ("monstrosities") that resemble normal parts of very different animals (like pigs born with something like a short trunk). If a related wild pig species actually had a trunk, people might argue it first appeared as a monstrosity. However, I've looked hard and haven't found cases where a monstrosity in one species looks like a normal part of a closely related species – and only those cases would really tell us something about how new features might arise this way.

Even if such monstrous forms did appear in the wild and could reproduce (which isn't always possible), they would be rare. For them to survive and become common would require unusually lucky circumstances. Plus, in the first few generations, they would breed with normal individuals, and their unusual trait would almost certainly be blended away and lost. (I'll come back to how single, occasional variations might be preserved later).

Small Differences Between Individuals

Now let's talk about the individual differences we see everywhere. These are the many small differences between:

Offspring born from the same parents.

Individuals of the same species living in the same small area.

Nobody thinks all individuals of a species are exact copies, like from a mold. These small differences are extremely important for our theory. Why?

They are often inherited (everyone knows this).

They provide the raw material for natural selection to work on and build up over time, just like humans build up desired traits in domestic plants and animals by choosing individuals with small differences.

These differences often affect parts that scientists consider "unimportant." But I could show (with many examples) that parts everyone agrees are "important" (for how the body works or how we classify organisms) also vary sometimes between individuals of the same species.

I'm convinced that even the most experienced naturalist would be shocked at the amount of variation they could find evidence for, even in important body parts, if they collected information over several years, as I have.

Keep in mind:

Scientists who classify organisms (systematists) generally don't like finding variation in important characters, as it makes classification harder.

Not many people are willing to do the hard work of carefully examining internal organs and comparing them across many individuals of the same species.

You wouldn't expect the way main nerves branch off near the central nerve cluster (ganglion) of an insect to vary within the same species. You might think such changes only happen very slowly over long periods. Yet, Sir J. Lubbock found that these main nerves in Coccus insects vary quite a bit, almost like the random branching of a tree stem. He also found that muscles in the larvae (young stages) of some insects are not consistent.

Sometimes, scientists argue in circles. They might say important organs never vary. But then (as some have admitted), they define "important" organs as those parts that don't vary! From that viewpoint, you'll never find an important part varying. But from any other viewpoint, you certainly can find many examples.

Confusing Groups: "Protean" or "Polymorphic" Genera

One very confusing situation involves certain groups of organisms (genera) called "protean" or "polymorphic." In these groups, the species show an enormous amount of variation.

Examples include plants like brambles (Rubus), roses (Rosa), and hawkweeds (Hieracium), as well as certain types of insects and shelled animals (Brachiopods).

For many forms in these groups, scientists can't agree whether to call them distinct species or just varieties of one species.

Interestingly, some species within these variable groups do have fixed, clear characters.

Genera that are polymorphic in one country usually seem to be polymorphic in other countries too. Judging from fossil Brachiopods, they were also polymorphic in the past.

These facts are puzzling because they suggest this extreme variability isn't simply caused by different living conditions. I suspect that, at least in some cases, we are seeing variations that don't really help or harm the species. As a result, natural selection (which we'll discuss later) hasn't "grabbed onto" them and made them consistent or definite.

Other Kinds of Differences Within a Species

Besides variation, individuals of the same species can have big structural differences for other reasons:

Differences between males and females.

Different types (castes) of sterile workers in insects like ants and bees.

Differences between young (immature or larval) stages and adults in many simpler animals.

Cases of dimorphism (two distinct forms) and trimorphism (three distinct forms) in both animals and plants.

Mr. Wallace recently highlighted butterflies in Southeast Asia where females of certain species regularly appear in two or even three very different forms, with no intermediate versions connecting them.

Fritz Müller found even stranger cases in Brazilian crustaceans. Males of one type (Tanais) come in two forms: one has strong, differently shaped claws, the other has antennae covered in many more smelling-hairs.

Although these forms often aren't connected by intermediate steps now, they probably were in the past. Mr. Wallace described a butterfly on one island that shows a whole range of varieties linking two extremes; these extremes look very much like the two separate forms of a related dimorphic butterfly species found elsewhere.

Similarly, ant worker castes are usually distinct, but sometimes we find intermediate forms connecting them. I've seen this in some dimorphic plants too.

It seems amazing at first that one female butterfly could produce three types of females plus a male, or that one plant could produce three types of flowers from the same seed pod! But really, these are just extreme versions of the everyday fact that females produce offspring of two sexes, and males and females can look dramatically different.

The Importance of "Doubtful Species"

Forms that look quite a bit like species, but are so similar to other forms, or so clearly linked to them by intermediate steps, that scientists hesitate to call them distinct species – these are very important for our argument.

We have good reason to think many of these doubtful and closely related forms have kept their characteristics stably for a very long time – perhaps as long as "good," undisputed species have.

How scientists decide: Practically, if a naturalist can find intermediate links connecting two forms, they usually treat one as a variety of the other. The form that is more common, or was described first, often gets ranked as the "species," and the other as the "variety."

Difficulties: But deciding is sometimes very hard, even when intermediate links exist (arguments about whether the intermediates are hybrids complicate things).

Guesswork: Very often, however, one form is called a variety of another not because intermediates have actually been found, but because scientists assume (based on analogy with other groups) that links exist somewhere now, or existed in the past. This opens the door wide for doubt and guesswork.

Who Decides? Relying on Expert Opinion

So, when deciding whether to call a form a species or a variety, the opinion of experienced naturalists with good judgment seems to be the only guide. Even then, we often have to go with the majority opinion, because almost every well-known variety has been ranked as a distinct species by at least some qualified expert at some point.

Doubtful Forms Are Common

There's no question that these kinds of doubtful forms are common.

Just compare lists of plants (floras) for places like Great Britain, France, or the United States made by different botanists. You'll see a surprising number of forms that one expert calls a species, and another calls a mere variety.

Mr. H. C. Watson (who helped me greatly) identified 182 British plants usually considered varieties, but which have all been ranked as species by some botanist. (He left out many minor varieties and very complex groups).

In highly variable plant groups, one expert (Mr. Babington) listed 251 species, while another (Mr. Bentham) listed only 112 – a difference of 139 doubtful forms!

Among animals that move around and mate freely, you rarely find doubtful forms (species vs. variety) living side-by-side in the same country. But they are common when comparing animals from different, separated areas.

Think about birds and insects in North America versus Europe. How many slightly different forms have been called definite species by one expert, and varieties (or "geographical races") by another?

Mr. Wallace studied animals (especially butterflies) on the islands of Southeast Asia. He found they could be grouped into:

Variable forms: Vary a lot within one island.

Local forms: Fairly consistent on each island, but when you compare them across islands, the differences are slight and gradual, making them hard to define, though the extremes look distinct.

Geographical races (or sub-species): Local forms that have become completely fixed and isolated. But they don't differ by major characters, so deciding if they are species or varieties is purely a matter of opinion.

Representative species: Fill the same ecological role as local forms/sub-species on different islands, but differ more significantly, so they are almost always called true species.

Wallace concluded there's no definite way to tell these categories apart.

More Examples of Blurry Lines

Years ago, comparing birds from the nearby Galapagos islands with each other and with mainland birds, I was struck by how vague and arbitrary the line between species and variety is.

On the small Madeira islands, Mr. Wollaston described many insects as varieties that other experts would certainly call distinct species.

Even Ireland has a few animals now usually seen as varieties but previously ranked as species.

Experienced bird experts consider the British red grouse just a strong variety of a Norwegian species, but most rank it as a unique British species.

Distance Matters: When two doubtful forms live far apart, scientists are more likely to call them distinct species. But how far is far enough? If America to Europe is enough, what about Europe to the Azores, Madeira, or Canary Islands? Or between the different small islands within those groups?

Walsh's "Plant-Eating" Varieties and Species

Mr. B. D. Walsh, an American insect expert, described what he called Phytophagic (plant-eating) varieties and species.

Most plant-eating insects stick to one or a few related plants. Some eat many kinds without changing.

But in several cases, Mr. Walsh found that insects living on different plants showed slight but consistent differences (in color, size, body fluids) in their larval or adult stages (or both). Sometimes only males differed, sometimes both sexes.

When the differences are more noticeable and affect all life stages and both sexes, everyone calls them good species.

But nobody can say for sure where to draw the line between these "phytophagic" varieties and species. Mr. Walsh suggested calling them varieties if they could probably still interbreed, and species if they seemed to have lost that ability.

Since the differences arise from feeding on specific plants for a long time, we wouldn't expect to find intermediate forms linking them now. This removes the best clue scientists usually have for deciding between variety and species. The same problem occurs with related organisms living on different continents or islands.

Conversely, if an animal or plant lives across a whole continent or many islands in a chain, and shows different forms in different areas, there's a good chance intermediate forms will be found, linking the extremes. In that case, the different forms are usually demoted to varieties.

Some Say Animals Don't Have Varieties...

A few scientists claim animals never have varieties. But these are the same scientists who consider the slightest difference enough to make a new species. If they find the exact same form in two distant places or two different geological layers, they assume they are two distinct species that just look identical! Under this view, the term "species" becomes a useless idea, just assuming separate creation for everything.

The reality is: many forms considered varieties by top experts look so much like species that other top experts have ranked them as such. Arguing about whether to call them species or varieties before we even agree on what those terms mean is pointless – like "beating the air."

Why Doubtful Cases Matter

These cases of strongly-marked varieties or doubtful species are worth thinking about. Scientists have used arguments from geographical location, similar patterns of variation in other groups, success of cross-breeding (hybridism), and more to try and figure out their rank. (I don't have space to discuss these arguments here).

More research will surely help scientists agree on how to classify many doubtful forms.

However, it's striking that we find the most doubtful forms in the countries we know best.

Also, notice this: if any wild animal or plant is very useful to humans, or gets a lot of human attention for any reason, varieties of it are almost always found and recorded. And often, some experts will rank these varieties as distinct species.

Look at the common oak tree. It's been studied closely. Yet one German scientist made over a dozen species out of forms that most other botanists consider mere varieties. In Britain, top botanists and practical foresters disagree on whether the two main types of oak (sessile and pedunculated) are distinct species or just varieties.

De Candolle's Oak Study: A Powerful Example

A. de Candolle recently published a major study of all the world's oaks. He had vast amounts of material and worked with great skill.

He first listed all the features that vary among oak species and how often they vary. He found over a dozen traits that can vary even on the same branch (sometimes due to age, sometimes for no clear reason). These traits aren't useful for defining species, but they are the kinds of traits often used in species descriptions.

De Candolle then defined "true species" as forms that differ in ways that never vary on the same tree and are never connected by intermediate forms.

After all this work, he stated forcefully: People who say most species are clearly defined and doubtful ones are rare are mistaken. That only seemed true when we didn't know groups well and based species on few samples. As we learn more, intermediate forms appear, and doubts about where species begin and end increase.

He also added that the best-known species are the ones with the most natural varieties and sub-varieties. For example, the common European oak (Quercus robur) has 28 varieties, mostly clustered around three main sub-species. The forms connecting these sub-species are relatively rare. If these rare connecting forms died out completely, the three sub-species would look exactly like distinct species (like some other closely related oak "species" that are currently recognized).

Conclusion: De Candolle admitted that out of 300 oak species to be listed in his big work, at least two-thirds are "provisional species" – meaning they aren't strictly known to meet his definition of a true species. He also came to believe species are not unchanging creations, concluding that the theory of evolution ("derivative theory") is more natural and fits better with known facts from fossils, geography, anatomy, and classification.

The Learning Process for a Scientist

When a young scientist starts studying a new group of organisms, they are initially confused about what counts as a species difference versus a variety difference. They don't know how much or what kind of variation is typical for that group. This confusion itself shows that variation is very common!

If they focus on one type of organism in one region, they'll soon start making decisions. They will tend to create many species, because, like the pigeon breeder, they become impressed with all the differences they constantly see. They lack broader knowledge of variation patterns in other groups and places to correct their initial view.

As they study more widely, they encounter more difficult cases because they find more closely related forms.

If they study very widely, they eventually form their own opinions, but usually only by accepting that a lot of variation exists. Other scientists might dispute this acceptance.

When they study related forms from places that are no longer connected (like different continents), where finding intermediate links is impossible, they have to rely almost entirely on analogy (comparing patterns), and the difficulties become extreme.

The Blurry Line: From Individual Difference to Species

It's clear that no sharp line has been drawn between:

Species and sub-species (forms very close to species rank).

Sub-species and well-marked varieties.

Lesser varieties and individual differences.

These categories blend into each other through tiny steps (an insensible series). Seeing such a series makes the mind think of an actual transition or passage from one state to another.

My View: Variation as Steps Towards Species

Therefore, I see:

Individual differences (though minor to classifiers) as highly important – the first steps toward slight varieties barely worth recording.

Slightly distinct varieties as steps toward more distinct and permanent varieties.

Strongly-marked varieties as steps leading to sub-species.

Sub-species as steps leading to species.

The change from one stage to the next might sometimes be just the result of the organism's nature and its environment over time. But for the more important changes, especially those that help the organism survive (adaptive characters), the transition from one stage to the next is likely due to:

The accumulating effect of natural selection (explained later).

The effects of increased use or disuse of body parts.

So, a well-marked variety can be called an incipient species – a species in the process of forming. Whether this view is correct depends on all the evidence presented in this work.

Not All Varieties Become Species

We don't need to assume that all varieties eventually reach the rank of species.

They might go extinct.

They might remain as varieties for very long periods (Mr. Wollaston showed this for fossil land snails in Madeira; Gaston de Saporta showed it for plants).

If a variety becomes more numerous than its parent species, it might then be called the species, and the original parent form called the variety.

Or, the variety might replace and exterminate the parent species.

Or, both might survive and eventually be ranked as independent species.
(We'll revisit this).

Species and Variety: Terms of Convenience

From all this, I see the term "species" as just a name given arbitrarily, for convenience, to a group of individuals that closely resemble each other. It doesn't represent something fundamentally different from the term "variety," which we give to less distinct and more changeable forms. "Variety," in turn, is also applied arbitrarily compared to mere individual differences.

Which Species Vary the Most? Looking for Patterns

Based on my theory, I thought I could find interesting patterns by counting varieties listed in well-studied plant catalogues (floras). It seemed simple at first, but Mr. H. C. Watson and Dr. Hooker quickly showed me there were many difficulties. (I'll discuss these and show the detailed numbers in a future work). Dr. Hooker, after reading my work and checking the tables, agrees the following statements seem well-supported. (This topic is complex and briefly treated here, so it involves ideas like "struggle for existence" that will be explained later).

Pattern 1: Wide-Ranging Species Have More Varieties
Alphonse de Candolle and others showed that plants with very large geographical ranges generally have varieties. This makes sense:

They are exposed to diverse physical conditions across their range.

They compete with different sets of other organisms in different places (which is perhaps even more important, as we'll see).

Pattern 2: Common and Widespread Species Vary Most ("Dominant" Species)
My tables also show that within any specific country:

Species that are most common (have the most individual plants/animals).

Species that are most widely spread within that country (which is different from having a huge overall range, and slightly different from just being common).
...are the ones that most often produce varieties distinct enough to be recorded by scientists.

Therefore, it's the most flourishing species, which we can call dominant species (wide-ranging, widespread locally, numerous individuals), that most often produce well-marked varieties, which I consider incipient species (species in the making).

Why this might be expected: Varieties have to struggle and compete to survive and become established. Species that are already dominant (successful) are the most likely to produce offspring that inherit their parents' advantages. These offspring might be slightly modified (varieties), but they still have a good chance of succeeding against their neighbors.

(Note on "dominant": This comparison only applies to forms competing with each other, especially members of the same group (genus or class) with similar lifestyles. When comparing numbers, we only compare within the same group. A widespread weed is dominant compared to other plants, but not necessarily compared to a vastly more numerous fungus or algae. But if that fungus exceeds its fungal relatives, it's dominant within its class.)

Pattern 3: Species in Larger Genera Vary More Often

Think about all the plants listed in a regional Flora (plant catalogue). Divide them into two groups:

Group 1: All species belonging to larger genera (genera that contain many species).

Group 2: All species belonging to smaller genera.

You'll find that Group 1 (larger genera) includes a slightly higher number of the very common, widespread, dominant species.

Why expected: If a genus has many species living in a country, it suggests something about the conditions there favors that whole group. So, we might expect the larger genera to have more dominant species. (This result is often hidden by other factors, like specific habitats (freshwater plants often range widely regardless of genus size) or how complex the organism is (simpler organisms often range more widely)).

Hypothesis & Test:

Idea: If species are just strongly-marked varieties, then in places where many related species have formed (i.e., in larger genera), we should also expect to find many varieties (incipient species) forming now. Where you find many large trees, you expect to find young saplings. If variation led to many species in a genus, conditions were likely favorable for variation, and might still be. (If species were special creations, there's no obvious reason why groups with more species should also have more varieties).

Test: I took plant lists from 12 countries and beetle lists from 2 districts. I divided the species into two groups: those in larger genera and those in smaller genera.

Result: In every single case, a larger proportion of the species in the larger genera had recorded varieties, compared to species in the smaller genera. Furthermore, the species in large genera that did have varieties had, on average, more varieties each than the species in small genera. These results held true even when I removed the very smallest genera from the comparison.

Meaning: This strongly supports the view that species originate as varieties. Where the "species manufactory" has been active (creating many species in a genus), it generally seems to be still active (creating many varieties, or incipient species). This makes sense because we believe making new species is a slow process. It doesn't mean all large genera are currently growing, or no small genera are (geology shows genus sizes change over time). It just means that, on average, where many species have been formed, many are still forming.

Other Connections Between Large Genera and Varieties

Species in Large Genera Resemble Varieties (Small Differences): We know there's no perfect way to tell species from well-marked varieties. When intermediate links are missing, scientists judge based on the amount of difference, using analogy to decide if it's "enough" for species rank. So, the amount of difference is important. Fries (for plants) and Westwood (for insects) noted that within large genera, the difference between the species themselves is often very small. My own calculations (though imperfect) and discussions with experienced scientists support this. In this way, species in larger genera resemble varieties – they differ from each other less than species in smaller genera typically do. Or, you could say: in the larger genera (where more varieties are currently being made), many of the existing species still look a bit like varieties because the differences between them are smaller than usual.

Species in Large Genera Cluster Like Varieties: Species within a genus are not all equally distinct. They are usually grouped into sub-genera or sections – little clusters of species around other central species (as Fries noted). What are varieties? They are also groups of forms, unequally related to each other, clustered around their parent species. (Of course, the amount of difference between varieties is much less than between species – but we'll see later how the Principle of Divergence of Character can explain how small differences grow into larger ones).

Closely Related Species Have Restricted Ranges (Like Varieties): Varieties generally have small geographical ranges (if a variety ranged wider than its parent, we'd just swap their names). There's evidence that species which are very closely related to other species (and thus resemble varieties) also often have restricted ranges. Mr. H. C. Watson analyzed British plants: 63 forms ranked as species but considered doubtfully distinct had average ranges similar to 53 acknowledged varieties. Both groups had much smaller ranges than the parent species to which the varieties belonged.

Summary of Analogies:

Varieties can't be perfectly distinguished from species (except by intermediates or an arbitrary amount of difference).

Species in larger genera have more varieties, on average.

Species in large genera are often closely but unequally related, forming clusters (like varieties around a parent).

Species very closely allied to others tend to have restricted ranges (like varieties).

Conclusion: These strong similarities between species in large genera and varieties make perfect sense if species originate as varieties. They are completely unexplainable if species are independent creations.

We've also seen it's the most flourishing (dominant) species within the larger genera that produce the most varieties. And varieties, as we'll see, tend to become new, distinct species. So:

Larger genera tend to become larger.

Dominant forms of life tend to become even more dominant by leaving many successful, modified descendants.

(However, larger genera also tend to break up into smaller ones over time – explained later).

This whole process leads to the pattern we see in nature: forms of life are divided into groups within groups, like a branching tree.

Life's Challenges: The Struggle for Existence

Before we dive into how nature chooses which variations survive (Natural Selection), let's talk about something crucial: the Struggle for Existence.

Connecting Variation to Nature's Big Picture

In the last chapter, we saw that individual plants and animals in the wild do vary. There are small differences between individuals, and sometimes more distinct varieties appear. Nobody really disputes this basic fact. It doesn't matter much for now whether we call slightly different forms "species," "sub-species," or "varieties."

But just knowing that variation exists doesn't explain how life becomes so wonderfully adapted. How did nature perfect things like:

A woodpecker's beak and tongue, perfectly suited for getting insects from trees?

Mistletoe, perfectly adapted to live on certain trees?

A tiny parasite clinging perfectly to an animal's fur or a bird's feathers?

A beetle's body, shaped just right for diving through water?

A seed with plumes designed to be carried by the slightest breeze?

We see these amazing adaptations everywhere. How did they happen?

Also, how do those varieties (which I called "incipient species" or species-in-the-making) eventually become distinct, separate species that are clearly different from each other, often much more different than simple varieties are? And how do whole groups of related species (called genera) arise, which differ even more?

The answer to all these questions involves the Struggle for Life.

Survival of the Fittest: Natural Selection

Because life is a struggle, any variation – no matter how small – that gives an individual an edge in its incredibly complex world will help that individual survive. These helpful variations will usually be passed down to its offspring.

Those offspring will then also have a better chance of surviving. Why? Because many individuals are born, but only a small number can actually live and reproduce.

I call the principle by which useful variations are preserved Natural Selection. This highlights its similarity to how humans select traits in domestic plants and animals. However, Mr. Herbert Spencer's phrase, the Survival of the Fittest, is often more accurate and just as useful.

We know humans can achieve amazing results through selection, shaping organisms for our own uses by accumulating small, useful variations that nature provides. But Natural Selection, as we'll see, is a force constantly at work. It is vastly more powerful than our weak human efforts, just as nature's works are far grander than anything humans create.

Understanding the Struggle

Let's look more closely at this struggle for existence. (I plan to cover this vital topic in much more detail in a future book).

Early scientists like De Candolle and Lyell clearly showed that all living things face intense competition. W. Herbert, using his vast gardening knowledge, wrote powerfully about competition among plants.

It's easy to say we believe in the universal struggle for life. But it's very difficult – at least I've found it so – to always keep this idea in mind. Yet, if we don't, the whole system of nature (why things live where they do, why some are rare and others common, why species disappear, how they change) remains confusing or misunderstood.

We look at nature and often see beauty and peace. We see plenty of food. We might forget:

The birds singing happily around us are often eating insects or seeds, constantly destroying other life.

These singers, their eggs, or their young are frequently killed by predators.

Even if food seems abundant now, it isn't always available throughout the year, every year.

What "Struggle for Existence" Means (It's Broad!)

I use the term "Struggle for Existence" in a wide, metaphorical sense. It includes:

Direct fighting: Two dogs fighting over scarce food are clearly struggling.

Dependence on conditions: A plant at the edge of a desert struggles against drought (though it's really dependent on moisture).

Competition for space/resources: A plant produces 1000 seeds, but only one grows to maturity. That seedling "struggles" against all the other plants already covering the ground.

Dependence between species: Mistletoe depends on apple trees. It's not really "struggling" with the tree (though if too many mistletoe plants grow, the tree dies).

Competition within a species: Several mistletoe seedlings growing close together on the same branch are truly struggling with each other.

Indirect competition: Mistletoe seeds are spread by birds. So, its existence depends on birds. You could say it "struggles" against other fruit-bearing plants to tempt birds to eat its berries and spread its seeds.

Most importantly, the struggle isn't just about staying alive; it includes success in leaving offspring. All these meanings blend together, and I use the general term "Struggle for Existence" for convenience.

Why the Struggle is Unavoidable: Rapid Multiplication

The struggle for existence happens automatically because all living things tend to reproduce at such a high rate.

Every organism that produces several eggs or seeds in its lifetime must face destruction at some point (some stage of life, some season, some years).

If not, its population would grow incredibly fast (geometrical increase), quickly becoming too large for any area to support.

Conclusion: Because more individuals are born than can possibly survive, there must be a struggle in every case.

This struggle can be:

Between individuals of the same species.

Between individuals of different species.

Against the physical conditions of life (climate, etc.).

This is the doctrine of Malthus (originally about human populations) applied much more strongly to the entire living world. For plants and animals, there's no artificial way to increase food supply, and no conscious choice to limit reproduction. Although some species might be increasing in number right now, all species cannot do so, or the world wouldn't be able to hold them.

Proof of Rapid Increase Potential

The Rule: There are no exceptions. If unchecked, the descendants of a single pair of any organism would soon cover the Earth.

Humans: Even humans, who reproduce slowly, have doubled their population in about 25 years. At that rate, in less than 1000 years, there literally wouldn't be enough space for everyone to stand.

Plants (Example): Linnaeus calculated that if a plant produced just two seeds per year (and no plant is that unproductive), and each seedling also produced two, and so on, there would be a million plants in only 20 years.

Elephants (Slowest Breeder Example): I estimated the slowest likely rate for elephants. Assume they start breeding at 30, stop at 90, have 6 young in total, and live to 100. Even with this slow rate, after about 740-750 years, one starting pair would have nearly 19 million living descendants.

Real-World Evidence is Stronger:
We have better proof than just calculations:

Animals in Favorable Conditions: Many recorded cases show animals increasing incredibly fast in the wild when conditions are good for a few seasons.

Domestic Animals Gone Wild: Even more striking is how fast domestic animals (like cattle and horses in South America and Australia) have multiplied when they escaped into the wild. The rates are unbelievable but well-documented.

Introduced Plants: Plants introduced to new places can sometimes take over whole islands in less than ten years. For example, European weeds like the cardoon and a tall thistle now cover huge areas of the plains in South America, pushing out native plants. Plants from America have spread all across India since its discovery.

In these cases, nobody thinks the animals' or plants' basic ability to reproduce suddenly increased. The simple explanation is:

Living conditions were very favorable.

Fewer individuals (old and young) died.

Almost all the young survived long enough to reproduce.
Their potential for geometrical increase simply explains their astonishingly rapid spread in their new homes.

Destruction Keeps Numbers in Check

In nature:

Almost every adult plant produces seeds each year.

Very few animals don't mate each year.

So, we can confidently say:

All plants and animals are trying to increase geometrically.

All would quickly fill every possible place they could live.

This tendency must be stopped by destruction at some point in their lives.

Our familiarity with large farm animals can mislead us. We don't see massive natural die-offs because we manage them. We slaughter thousands for food annually. In nature, an equal number would have to die somehow.

The only difference between organisms that produce thousands of eggs/seeds and those that produce very few is how long it takes them to fill an area under ideal conditions. Slow breeders just need more years.

The condor (2 eggs) might be more numerous in an area than the ostrich (20 eggs).

The Fulmar petrel (1 egg) is thought to be possibly the most numerous bird on Earth.

One fly lays hundreds of eggs, another lays just one; this doesn't determine how many flies can live in an area.

Having many eggs or seeds helps species whose food supply changes a lot, allowing them to boom when food is plentiful. But the main importance of large numbers of eggs/seeds is to make up for heavy destruction that happens at some life stage (usually early on).

If parents can protect their eggs or young well, fewer need to be produced.

If many eggs or young are destroyed, many must be produced, or the species will die out.

A tree living 1000 years would only need to produce one single seed in its lifetime to maintain its numbers, if that seed was guaranteed never to be destroyed and to land in a perfect spot to grow.

So, the average number of any animal or plant depends only indirectly on how many eggs or seeds it produces. It depends more on how many survive the checks.

Always Remember This:

Every living being is striving to multiply.

Every living being faces a struggle at some point.

Heavy destruction will happen (to young or old) in each generation or periodically.

If you reduce any check, even slightly, the species' numbers will shoot up almost instantly.

What Stops Unlimited Growth? The Checks

The specific causes that limit population growth are mostly unknown. Even for the most successful, abundant species, we don't know exactly what keeps them in check. This isn't surprising, given how little we know about population checks even for humans, whom we understand far better than any other animal. (Many writers have studied this, and I plan to discuss it more, especially regarding wild animals in South America).

Here are just a few key points about these checks:

Life Stage: Eggs and very young animals often suffer the most death, but not always.

Plants: Huge numbers of seeds are destroyed. My observations suggest seedlings suffer most from competition when they sprout in ground already full of other plants. Seedlings are also eaten in vast numbers (e.g., on a small patch of cleared ground I watched, slugs and insects killed 295 out of 357 weed seedlings). Competition also kills older plants (e.g., on a small patch of mown lawn I let grow wild, 9 out of 20 plant species died as the stronger ones grew tall).

Food Supply: This sets the absolute upper limit on population size.

Predation (Being Eaten): Very often, being prey for other animals is what determines the average number of a species. For example, the number of partridges, grouse, and hares on large estates seems mainly controlled by the number of predators ("vermin"). If hunting stopped in England for 20 years, but predator control also stopped, there would probably be less game than now, even though hunters kill hundreds of thousands annually. Conversely, some animals, like elephants, have almost no predators.

Climate: Plays a big role in average numbers. Extreme cold or drought seems to be the most powerful check. I estimated the severe winter of 1854-5 killed about 80% (four-fifths) of the birds on my property – a massive die-off compared to human epidemics.

Indirect Effects of Climate: Climate often acts indirectly. It might reduce the food supply, which then causes the most severe struggle between individuals (of the same or different species) that eat the same food.

Direct Effects of Climate: Even when extreme cold acts directly, it kills the weakest individuals – those who couldn't find enough food as winter progressed.

Range Limits and Competition: As you travel towards colder or drier regions, you see species become rarer and finally disappear. It's tempting to blame the climate directly. But this is often wrong. Even where a species is most common, it's constantly suffering huge losses from enemies or competitors. If a slight climate change favors these enemies or competitors even a little, they will increase. Since the area is already full, the original species must decrease. When a species decreases towards the south (warmer/wetter), the cause is likely other species being favored. When it decreases towards the north (colder/drier), other species being favored is still a factor, but direct harm from the climate is more likely (since there are fewer species, and thus fewer competitors, further north or higher up mountains – this is why we see more stunted plants in these areas). Only in extreme environments (Arctic, high mountains, deserts) is the struggle almost entirely against the physical elements.

Garden Plants Prove Indirect Effects: Many foreign plants can survive our climate perfectly well in gardens but never spread into the wild. Why? They can't compete with our native plants or survive attacks from our native animals.

Disease (Epidemics): When a species becomes extremely numerous in one area (often happens with game animals), epidemics frequently break out. This seems like a check independent of competition. However, some "epidemics" might actually be caused by parasitic worms that spread easily among crowded animals. This then becomes a struggle between the parasite and its host.

Safety in Numbers: On the other hand, sometimes having a large population is essential for a species' survival, especially relative to its enemies.

We easily grow crops like corn because we plant so many seeds that the birds can't possibly eat them all. The bird population can't increase enough during that season to match the food supply because their numbers are limited during winter.

But anyone who tries to grow just a few wheat plants in a garden knows how hard it is to save any seeds from the birds.

This need for large numbers might explain why very rare plants are sometimes extremely abundant in the few spots where they do manage to live. It might also explain why "social" plants (that grow in dense groups) remain social even at the very edge of their range. Perhaps they can only survive where conditions allow many to live together, protecting the species from being wiped out entirely. (Good effects of cross-breeding and bad effects of inbreeding are likely involved here too).

Nature's Tangled Web: Complex Relationships

The checks and relationships between struggling organisms are often incredibly complex and unexpected.

Example 1: Heathland and Pine Trees: I studied a large, barren heath in Staffordshire that had never been altered by humans. Nearby, several hundred acres of identical heath had been enclosed 25 years earlier and planted with Scotch fir trees. The change in the planted area was amazing:

The types and numbers of native heath plants completely changed.

Twelve new plant species (not including grasses) thrived in the pine area but weren't on the open heath.

The effect on insects must have been even greater, as six types of insect-eating birds were common in the pines but absent from the heath (which had 2-3 different insect-eating birds).

Conclusion: Introducing just one type of tree, plus fencing out cattle, dramatically changed the entire ecosystem.

Example 2: Heathland and Cattle: Near Farnham, Surrey, I saw the importance of fencing (enclosure). Large heaths had only a few old clumps of Scotch firs on distant hills. But inside large areas fenced off in the last 10 years, young pines were sprouting up everywhere, planted by nature (self-sown). I was surprised and checked the vast unenclosed heath – not a single young pine visible! But looking closely among the heather, I found countless tiny pine seedlings that had been constantly eaten down by cattle. In one square yard, I counted 32 little trees; one had 26 growth rings but was still tiny, having failed for years to grow taller than the heather. Conclusion: As soon as the land was fenced, the pines thrived. Cattle were completely determining whether pines could grow on that huge, barren heath, even though you wouldn't think they'd find enough food there to search so thoroughly.

Example 3: Flies, Cattle, Birds, and Plants: Cattle determine the existence of pines in Surrey. But elsewhere, insects can determine the existence of cattle! In Paraguay, cattle, horses, and dogs haven't established wild populations (though they do nearby). This is because of a specific fly that lays eggs in the navels of newborns, killing them. The fly numbers must be checked by something, probably other parasitic insects. So, imagine this chain reaction:

If certain insect-eating birds decreased...

The parasitic insects that feed on the navel fly might increase...

This would cause the navel flies to decrease...

Then cattle and horses could survive and run wild...

This would drastically change the vegetation (as I've seen elsewhere)...

Changing vegetation would affect other insects...

Which would affect the insect-eating birds...
...and so on, in ever-widening circles of complexity.

Nature is rarely that simple. There are constant battles within battles, with shifting success. Yet, over the long run, the forces balance so well that nature appears stable for long periods. A tiny change, however, could give one organism victory over another. We are so ignorant of these complex relationships, yet so arrogant, that we are surprised when a species goes extinct. We blame catastrophes or invent imaginary laws about lifespan instead of understanding the underlying causes.

Example 4: Cats, Mice, Bees, and Flowers: Here's one more example showing how distant organisms are connected.

The exotic Lobelia fulgens flower in my garden never gets visited by insects and thus never produces seed (due to its structure).

Most orchids absolutely need insects to move their pollen for fertilization.

Experiments show bumblebees are essential for fertilizing pansies (Viola tricolor).

Bees are also needed for some clovers. I protected 20 heads of Dutch clover from bees – zero seeds produced (compared to 2,290 seeds from 20 unprotected heads). 100 heads of red clover protected from bees – zero seeds (compared to 2,700 from 100 unprotected heads). Only bumblebees visit red clover (others can't reach the nectar).

Conclusion: If bumblebees disappeared from England, pansies and red clover might become very rare or disappear too.

Bumblebee numbers largely depend on field mice, which destroy their nests (Col. Newman believes mice destroy over two-thirds of nests).

Mouse numbers largely depend on cats. Col. Newman found more bee nests near villages (where there are more cats to kill mice).

Chain: Therefore, it's entirely believable that the number of cats in an area could determine how common certain flowers are, through the chain reaction involving mice and then bees!

Summary of Checks and Balances

Every species faces many different checks (predators, climate, disease, competition, etc.).

These checks act at different life stages, seasons, or years.

Usually, one or a few checks are the most powerful, but all contribute to determining the average population size, or even if the species can exist there at all.

Different checks can affect the same species in different regions.

When we look at a tangled bank of plants, their types and numbers seem random ("chance"). But this is wrong! Think of the cleared Indian ruins now covered in diverse forest like the surrounding untouched areas. What centuries of struggle occurred there! Trees competing, insects warring, animals eating plants and each other – all striving to increase, determining the final balance. It's far more complex than feathers falling according to simple laws.

The Struggle is Fiercest Between the Closest Relatives

Dependence: Often occurs between distantly related beings (like a parasite and its host).

Struggle: Can also occur between distant beings (like locusts and grass-eating mammals competing for food).

Most Intense Struggle: Is almost always between individuals of the same species. Why? They live in the same areas, need the same food, and face the same dangers.

Varieties of the Same Species: The struggle between varieties is generally almost as severe. We see this resolved quickly sometimes:

If you sow different wheat varieties together and replant the mixed seed, the varieties best suited to the conditions, or naturally most fertile, will produce more seed and quickly replace the others.

To keep a mix of different colored sweet peas, you must harvest seeds separately each year and then mix them; otherwise, the weaker types vanish.

Some mountain sheep varieties will starve out others if kept together.

Different varieties of medicinal leeches also compete, with some eliminating others.

It's doubtful if any mix of domestic varieties (if crossing is prevented) could maintain its original proportions for even six generations if left to struggle naturally without human intervention.

Species of the Same Genus: Since species in the same genus usually have similar habits and needs (and always similar structures), the struggle between them will generally be more severe than the struggle between species from different genera, if they compete. Examples:

One swallow species spreading in the US caused another to decrease.

The missel-thrush increasing in Scotland caused the song-thrush to decrease.

One rat species often replaces another worldwide.

An Asiatic cockroach drove out a larger native one in Russia.

Imported European honeybees are wiping out small native stingless bees in Australia.

One species of charlock weed replaced another.

We can vaguely understand why competition is toughest between similar forms occupying similar ecological roles, but we probably couldn't explain the exact reason for victory in any single case.

The Most Important Conclusion: Everything is Connected

From all this, we can deduce something highly important: The structure of every living thing is related, in the most essential yet often hidden ways, to that of all other living things it interacts with. This includes those it:

Competes with (for food, space, etc.).

Escapes from (predators).

Preys upon.

This connection is obvious in a tiger's teeth and claws, or a parasite's legs and claws. It's less obvious, but just as true, in:

A dandelion's plumed seed: Seems related only to the air. But its real advantage is helping seeds spread widely to find open ground, because the land is already crowded with competing plants.

A water beetle's flattened legs: Seems related only to water. But they allow it to dive efficiently, competing with other water insects, hunting its prey, and escaping being eaten itself.

Even the food stored inside a seed (like in peas and beans) seems unrelated to other organisms at first. But the strong growth of seedlings from these large seeds, especially when sown among tall grass, suggests the stored food's main purpose is to help the seedling survive its initial struggle with surrounding plants.

Imagining Change and Our Ignorance

Think about a plant living happily in the middle of its range. Why doesn't it double or quadruple its numbers? We know it can handle slightly warmer, colder, wetter, or drier conditions because it lives in such places elsewhere in its range. So, the limit isn't just climate tolerance here. To help this plant increase here, we'd need to give it an advantage over its current competitors or the animals that eat it.

Now consider the plant at the very edge of its range. Here, a change in its basic constitution to better handle the climate would clearly help. But we believe only a few plants or animals range so far that they are killed only by climate harshness. Competition usually doesn't stop until you reach the absolute extremes of life (Arctic ice, true deserts). Even there, a few species will compete for the warmest or wettest spots.

Therefore, when a plant or animal is moved to a new country with new competitors and enemies, the conditions of its life change fundamentally, even if the climate is identical to its old home. To help it increase its numbers in the new home, we'd have to modify it differently than we would have in its native country, giving it advantages over a different set of rivals.

It's a good exercise to try, in your imagination, to give one species an advantage over another. You'll probably find you wouldn't know what specific change to make. This should impress upon us how ignorant we truly are about the complex relationships between all living things. Realizing this ignorance is as necessary as it is difficult.

A Final Thought: The Brighter Side of Struggle

All we can do is keep firmly in mind:

Every living thing is trying to increase exponentially.

Every living thing, at some point, struggles for life and suffers great destruction.

When we think about this constant struggle, we can take comfort in the belief that:

The "war of nature" is not non-stop violence.

Animals likely don't live in constant fear.

Death, when it comes, is generally quick.

And ultimately, it is the vigorous, the healthy, and the happy that survive and multiply.

How Nature Chooses: Natural Selection

We've seen that life is a struggle for existence. How does this struggle affect the variations we see in nature? Can the idea of selection, which humans use so effectively, also apply in the wild? I believe it can, and very powerfully.

Remember these points:

Living things have countless small variations and individual differences, especially domesticated ones, but also wild ones.

Traits tend to be passed down (inheritance).

Under human care, organisms become somewhat flexible or "plastic."

But humans don't create these variations, as scientists like Hooker and Asa Gray noted. We can only preserve and add up the variations that happen naturally. We unintentionally cause variation by changing living conditions, but similar changes happen in nature too.

Also, remember how incredibly complex the relationships are between all living things and their environment. This means that almost any kind of variation in structure could potentially be useful under changing conditions.

So, ask yourself:

Since variations useful to humans have definitely occurred, isn't it likely that variations useful to the organism itself, in the great battle of life, also occur over many generations?

If useful variations do happen, can we doubt that individuals with even a slight advantage over others would have the best chance of surviving and having offspring? (Remember, far more are born than can possibly survive).

On the flip side, we can be sure that any variation that is even slightly harmful would be quickly eliminated.

This process – the preservation of helpful differences and variations, and the getting rid of harmful ones – I call Natural Selection, or the Survival of the Fittest.

What about variations that are neither helpful nor harmful? Natural selection wouldn't affect them. They might remain as fluctuating traits (like maybe we see in highly variable "polymorphic" species), or they might eventually become fixed characteristics due to the organism's nature or its environment.

Clearing Up Misconceptions About Natural Selection

Some writers have misunderstood or objected to the term "Natural Selection":

Does it cause variation? No. Natural selection only preserves variations that already exist and happen to be beneficial in a particular environment. It's like farmers selecting the best variations nature provides; they don't create the variations themselves.

Does it imply conscious choice? No. Animals and plants don't consciously "choose" to change. The term is metaphorical, like when chemists talk about chemical "affinities" (one chemical "choosing" to combine with another). Nobody thinks the chemicals are actually choosing.

Is it a power or deity? No. When I talk about Natural Selection "acting," it's like talking about gravity "ruling" the planets. It's just a shorthand way of describing the outcome of natural laws. By "Nature," I simply mean the combined action and results of many natural laws (the observed sequence of events). With a little use, these kinds of misunderstandings should fade away.

How Natural Selection Might Work: A Scenario

Let's imagine how natural selection might proceed. Picture a country undergoing a slight change, maybe in climate.

The balance between its existing inhabitants would shift almost immediately. Some species might even go extinct.

Because all living things in an area are interconnected in complex ways, any change in the numbers of some species would seriously affect others, even without the direct climate change itself.

If the country had open borders, new species would definitely move in, further disturbing the existing relationships. (Remember how powerful introducing just one new tree or mammal can be).

But what if the area is isolated? Like an island, or a region partly blocked by barriers. Here, new, potentially better-adapted species couldn't easily enter. This would leave ecological "jobs" or niches that aren't filled as well as they could be.

In this situation, if any of the original inhabitants happened to vary in ways that made them slightly better suited to the new conditions, these variations would tend to be preserved. Natural selection would have a clear opportunity to "improve" the locals to fill those available roles.

Favorable Conditions for Natural Selection

Changing Conditions & Variability: We believe changes in living conditions tend to increase variation (as discussed in Chapter 1). In our scenario, conditions have changed, which would help natural selection by providing more variations to possibly work with.

Helpful Variations Must Occur: If no beneficial variations arise, natural selection can do nothing. Remember, "variations" always includes simple individual differences.

Time: Just as humans achieve results by accumulating small individual differences over generations, natural selection can do the same, but much more easily because it has vastly more time.

Open Niches Aren't Always Needed: I don't think big physical changes or complete isolation are always required for natural selection to work. Because the inhabitants of any area are constantly struggling in a delicately balanced system, even a very slight change in one species' structure or habits could give it an advantage over others. Further similar changes could increase that advantage.

Perfection is Never Reached: No country exists where all the native inhabitants are so perfectly adapted that no improvement is possible. We know this because in every country, introduced foreign species ("naturalized productions") have managed to take hold and compete successfully, sometimes driving out natives. If foreigners can beat natives, it proves the natives could have been modified to better resist them.

Nature's Power vs. Man's Efforts

If humans can achieve so much with our selection methods (both planned and unplanned), what can natural selection achieve?

Scope: Humans select based only on external, visible traits. Nature ("natural preservation") doesn't care about appearances unless they are useful. Nature can act on every internal organ, every subtle difference in constitution, the entire "machinery of life."

Goal: Humans select only for our own benefit. Nature selects only for the benefit of the organism itself.

Testing: Nature thoroughly tests every trait it selects, simply because survival is the test. Humans often don't test traits properly (e.g., we keep pigeons with different beak lengths on the same food; sheep with different wool types in the same climate).

Rigor: Humans prevent the strongest males from fighting for mates. We protect many inferior individuals. Nature involves a real struggle.

Starting Point: Humans often start selection with a noticeable variation, even a "half-monstrous" one. In nature, the tiniest difference can tip the balance in the struggle for life and be preserved.

Timescale: Human efforts are short-lived. Nature accumulates results over immense geological periods.

Conclusion: Is it any wonder that Nature's productions are far "truer" (more consistently adapted), infinitely better suited to complex life conditions, and clearly show signs of far superior "workmanship" compared to human-bred varieties?

Natural Selection: A Constant, Slow Process

Think of natural selection metaphorically: It's constantly, daily and hourly, scrutinizing every tiny variation across the globe. It rejects the bad, preserves and adds up the good. It works silently and slowly, whenever and wherever conditions allow, improving each living thing in relation to its environment (both living and non-living).

We don't see these slow changes happening in real time. Only after ages have passed does the "hand of time" reveal the results. And even then, our view into the deep past is so incomplete that we mainly just see that life forms today are different from those long ago.

Ongoing Variation is Needed

For significant change to happen in a species, a new variety, once formed, must eventually vary again in a similar helpful way. These new variations must again be preserved, and so on, step by step. Since we see individual differences constantly recurring, assuming this happens over long periods isn't unreasonable. Whether this hypothesis is true can only be judged by how well it explains the patterns we see in nature. (The common belief that variation is strictly limited is also just an assumption).

Selection Can Act on "Minor" Traits

Even traits that seem very unimportant to us can be acted upon by natural selection if they offer even a slight advantage.

Camouflage: Green insects on leaves, mottled-grey insects on bark, ptarmigan turning white in winter snow, red-grouse matching heather color. These colors surely help protect them from predators. Hawks hunt by sight (people on the continent were warned not to keep white pigeons because they are easily spotted). So, selection could give each grouse species its proper color and keep it consistent. Don't think occasional deaths don't matter; remember how crucial it is for sheep farmers to remove any lamb with a trace of black from a white flock. The color of pigs in Virginia determined if they lived or died from eating paint-root.

Plant Traits: Botanists consider fruit fuzz (down) or flesh color minor traits. Yet, horticulturist Downing noted in the US:

Smooth-skinned fruits suffer more from curculio beetles than downy ones.

Purple plums suffer more from one disease than yellow plums.

Another disease attacks yellow-fleshed peaches more than other colors.
If these small differences matter so much in cultivation (with human help), they would certainly be decisive in nature, where trees struggle against other trees and many enemies, determining which variety (smooth/downy, yellow/purple) succeeds.

Don't Forget Other Factors
When looking at small differences between species that seem unimportant:

Remember direct environmental effects (climate, food) might play a role.

Remember correlation: When one part changes due to selection, other parts will change too, often in unexpected ways.

Selection Can Act at Any Age

Variations that appear at a specific life stage tend to be inherited by offspring at that same stage. Examples: seed traits, silkworm larva/cocoon traits, poultry egg/chick traits, sheep/cattle horn traits (appear when nearly adult).

Implication: Natural selection can therefore modify organisms at any age by accumulating variations useful at that age and ensuring they are inherited at that age.

Example: If it helps a plant spread seeds by wind, selection could gradually improve the seed's downiness, just like a cotton farmer selects for more down.

Example: Selection can adapt an insect larva to its unique challenges, independently of the adult's needs (though correlation might link larval and adult structures). Adult modifications might affect the larva. But selection ensures changes aren't harmful overall, or the species would die out.

Scope of Natural Selection

Modifies young relative to parent, and parent relative to young.

In social animals, adapts individuals for the good of the community if the change benefits the group.

What it cannot do: Modify one species only for the benefit of another species, without giving the first species any advantage. (Claims like this exist but don't hold up to scrutiny).

Can modify structures used only once if crucial (insect jaws for emerging from cocoon, bird egg-tooth for hatching).

Example (Pigeon Beaks): Short-beaked tumbler pigeons often die in the egg. If nature needed to make adult pigeon beaks very short for the bird's own benefit, the process would be slow. It would involve rigorous selection of young inside the egg that had the strongest beaks to hatch. Or, selection might favor eggs with thinner, more easily broken shells (shell thickness varies too).

Random Destruction Happens

Remember, much destruction in nature is random or "fortuitous" and has little direct effect on natural selection.

Vast numbers of eggs/seeds are eaten each year. Selection could only protect them if they varied in a way that made them less likely to be eaten. Many eaten eggs/seeds might have produced individuals better adapted than those that survived by chance.

Many adults are killed by accidents (storms, floods) regardless of how well-adapted they are in other ways. Changes beneficial in other aspects wouldn't prevent this accidental death.

But selection still works: Even if destruction is heavy, as long as it doesn't completely wipe out the potential for increase, the best-adapted individuals among the survivors (assuming favorable variation exists) will tend to leave more offspring than the less-adapted. If numbers are totally kept down by random causes, selection might be powerless in certain directions, but it can still act effectively at other times or on other traits. We have no reason to think all species are constantly improving in all ways simultaneously anyway.

Sexual Selection: A Partner to Natural Selection

Since traits often appear in one sex and are inherited by that sex, it's possible for the two sexes to be modified differently by natural selection for different lifestyles. More commonly, one sex is modified in relation to the other sex through Sexual Selection.

Definition: Sexual selection isn't about survival against the environment or other species. It's about the struggle between individuals of one sex (usually males) to mate with the other sex.

Outcome: The loser doesn't necessarily die, but has few or no offspring. It's generally less intense than natural selection.

Who Wins? Often, the most vigorous males (best suited overall) leave the most offspring. But often, victory depends on special weapons only males have (horns on stags, spurs on cocks). Allowing the victor to breed could lead to traits like courage, long spurs, strong wings – similar to how cockfighters breed better fighters.

Examples of Male Combat: Alligators fighting, male salmon fighting all day, stag beetles wounding each other, male insects fighting while the female watches (observed by M. Fabre). Combat seems fiercest among polygamous animals (males mate with multiple females), which often have special weapons. Carnivores are already armed, but sexual selection might give them defensive extras (lion's mane, salmon's hooked jaw).

Peaceful Contests (Birds): Often involves rivalry to attract females through song or visual displays. Males congregate (Guiana rock-thrush, birds of paradise), show off gorgeous plumage, perform strange dances. Females watch and choose the most attractive partner. Birds in captivity show individual preferences (Sir R. Heron's pied peacock).

Result: If humans can breed beautiful bantams quickly based on our standards, there's no reason female birds, by choosing the most attractive males over thousands of generations based on their standards, couldn't produce significant effects on male appearance or song. Differences in plumage between sexes and young birds can partly be explained by sexual selection acting on variations appearing at different ages and being inherited accordingly.

Conclusion on Sexual Selection: When males and females live similarly but look different (color, structure, ornaments), these differences are likely caused mainly by sexual selection. Individual males had slight advantages (weapons, defense, charms) passed only to male offspring over generations. (However, not all sex differences are due to this; some sex-linked traits in domestic animals seem to arise without selection, like the wild turkey's breast tuft).

Illustrations of Natural Selection at Work

Let me give one or two imaginary examples to clarify how I think natural selection acts:

Wolves and Deer: Imagine wolves prey on various animals (using stealth, strength, or speed). Suppose their fastest prey (deer) becomes more common, or other prey becomes rarer, during the time of year when wolves struggle most for food.

In this situation, the swiftest and slimmest wolves would have the best chance of catching deer and surviving. These individuals would be preserved or "selected" (assuming they were still strong enough to handle other prey at other times).

I see no reason to doubt this outcome, just as humans can improve greyhound speed through careful selection (planned or unplanned).

(Note: There are apparently two wolf varieties in the Catskill Mountains, USA – a slender, greyhound-like one that hunts deer, and a bulkier one that attacks sheep flocks more often).

Important Clarification: I'm talking about the preservation of many individuals with slight advantages (slimmer wolves), not the preservation of a single, rare, strongly-marked variation (like a "monstrosity"). My earlier writing sometimes implied the latter. I now better appreciate (thanks to a review in the North British Review, 1867) how rarely single variations, even beneficial ones, can become permanent. The chances are strongly against a single advantageous individual surviving and its trait spreading widely without being diluted by crossing with normal individuals. However, the result would likely follow from preserving many individuals with slightly curved beaks (for example) and destroying many with straighter beaks, over generations.

Recurring Variations: Remember that certain variations often reappear because similar conditions act on similar organisms. Even if the trait itself isn't directly inherited, the tendency to vary that way is. Sometimes this tendency is so strong that all individuals change similarly without selection. Sometimes only a fraction changes (like the Uria lacrymans guillemot variety). If such a common variation is beneficial, natural selection (survival of the fittest) will quickly favor the modified form over the original.

Intercrossing and Local Varieties: Intercrossing tends to eliminate variations. But most organisms stay in their local areas. So, a new variety would likely start locally. The similarly modified individuals would live together and breed together. If successful, the variety would spread outwards from this central area, competing with and replacing the original form as it expands.

Flowers and Insects Co-evolving (More Complex):

Some plants produce nectar (sweet juice), maybe to get rid of waste sap. Insects eat it, but this doesn't initially help the plant.

Now, imagine nectar is produced inside the flowers of some plants. Insects visiting for nectar get dusted with pollen and carry it to other flowers.

This cross-pollination produces more vigorous seedlings, giving them a better survival chance.

Plants producing the most nectar attract the most insects, get crossed more often, and gain an advantage, forming a local variety.

Flowers whose stamens and pistils are arranged better for pollen transfer by specific insects would also be favored.

(Same logic applies if insects collect pollen: cross-pollination benefits the plant even if much pollen is lost).

Once insects regularly transfer pollen, division of labor becomes beneficial: separate male and female flowers/plants. Natural variations in sexual organ function, combined with the advantage of specialization, would be favored by selection until sexes might become completely separate. (Process visible today through dimorphism etc.; North American holly species show intermediate stages).

Now consider the insects: Suppose the plant becomes common, and certain insects rely heavily on its nectar. Bees are known to save time (e.g., biting holes in flower bases). Slight variations in proboscis length or shape could help a bee get nectar faster. Its community would thrive and produce more offspring with the same trait. (Example: Hive bees can suck nectar from incarnate clover but not red clover, though the tube length difference seems tiny. If red clover common, longer proboscis would greatly benefit hive bees).

Mutual Adaptation: Conversely, if bumblebees (needed for red clover) became rare, it would benefit the clover to have shorter tubes so hive bees could pollinate it.

Conclusion: A flower and its pollinator bee could slowly become perfectly adapted to each other, simultaneously or one after another, by the continuous preservation of individuals with slight structural changes beneficial to both.

Natural Selection is Like Slow Geological Change

My theory of natural selection acting through small, accumulated changes faces the same objections initially raised against Sir Charles Lyell's geological views (that slow, ongoing processes shape the Earth). We now accept that slow erosion carves valleys. Similarly, natural selection, acting only by preserving small useful modifications, will eventually replace the belief in constant creation of new species or sudden large changes in their structure.

A Detour: Why Crossing is Likely Universal

Separate Sexes: Obviously require two individuals to reproduce (except rare parthenogenesis).

Hermaphrodites (Both Sexes in One): Less obvious, but strong reason to believe all hermaphrodites cross with another individual, at least occasionally.

Breeding Evidence: Crossing different varieties/strains boosts offspring vigor/fertility; close interbreeding reduces it. This universal breeder experience suggests occasional crossing is a law of nature, needed to maintain fitness over generations.

Flower Structures Explained: Why are many flower sex organs exposed (risking damage)? To allow pollen from others in, vital if occasional cross needed. Why do enclosed flowers (pea family) have complex insect adaptations? Insects ensure crossing. (Plant's own pollen wins against different species' pollen, preventing unwanted hybrids).

Self-Fertilizing Mechanisms? Often need insect trigger (barberry) and don't prevent crossing. Many flowers have tricks to prevent self-pollination (Lobelia pollen swept out before stigma ready; dichogamy - sex organs mature at different times). Strange unless occasional cross is vital.

Mongrels from Crossing: Mixed varieties (cabbage, radish) produce mostly mongrel seedlings, showing cross-pollen is more effective ("prepotent") than self-pollen within the same species. Supports benefit of crossing.

Trees: Tendency towards separate sexes increases chances of cross-tree pollination.

Animals: No self-fertilizing terrestrial hermaphrodite found (snails, earthworms pair). Makes sense if cross needed, as no wind/insect equivalent for pollen transfer. Aquatic hermaphrodites can use water currents for occasional cross. Even enclosed ones (barnacles) found to cross sometimes.

Small Difference: If all hermaphrodites cross occasionally, functional difference from separate-sex species is small.

Conclusion: Based on many facts, an occasional cross seems a very general, if not universal, law of nature.

Circumstances Affecting Natural Selection's Success

This is complex. What helps or hinders natural selection in creating new species?

Favorable Factors:

Variability: More variation (including individual differences) is better.

Large Population: Very important. Increases chances of useful variations appearing. Compensates for less individual variation. Needed because time isn't infinite; species must improve or lose to competitors.

Inheritance: Favorable variations must be passed on.

Isolation: Can be important. Prevents crossing with outsiders, allows uniform modification in a confined area, prevents immigration of better-adapted forms, gives time for slow improvement.

Large Area (Likely More Important than Isolation): Especially for producing enduring, widespread species. Supports larger populations (more variation). More complex conditions (more competitors) drive improvement. New forms can spread widely. Often includes past isolation phases (due to land level changes). Modification likely faster, produces stronger competitors. (Explains perhaps why continental species dominate islands, why freshwater has "living fossils" due to less competition).

Time: Allows chances for variation, selection, accumulation, fixation. Allows direct environmental effects.

Unfavorable Factors:

Lack of Variation: If no useful variations arise, selection is powerless.

Reversion: Tendency to revert to ancestral forms can hinder progress.

Intercrossing: Can swamp out new variations, especially for mobile animals. Less problematic for local varieties, occasional crossers, or rapid breeders. (But doesn't always prevent distinct varieties co-existing via different habitats, breeding times, or mate preference). Benefit: Keeps species uniform. Essential for all via occasional crosses (boosts vigor).

Small Population (in small isolated areas): Reduces chances of favorable variations arising, slowing new species production.

Ideal Scenario (Summary): Large continent undergoing geological changes (becoming islands, then reuniting). Provides large populations, intense competition, periods of isolation for refinement, then renewed competition favoring best-adapted widespread forms.

Slowness and Power of Natural Selection

Slowness: I fully admit natural selection generally acts very slowly. Only works when niches can be better filled by modification. Depends on slow physical changes or prevention of immigration. Modifying some species disturbs others, slowly opening new niches. Takes time for right variations to occur. Intercrossing slows it.

Counter-Argument: Some say these factors negate selection's power. I disagree.

Conclusion: Selection is likely slow, intermittent, affecting few inhabitants at a time. This fits the geological record of how life changed.

Ultimate Power: Despite slowness, if weak humans achieve much by selection, there seems no limit to the change, beauty, and complex co-adaptations nature can achieve over vast time through survival of the fittest.

Extinction: An Inevitable Result

Extinction is closely tied to natural selection.

Selection preserves advantageous variations.

Areas are already full (due to geometric increase).

Therefore, as favored forms increase, less favored forms must decrease and become rare.

Rarity leads to extinction (geology confirms). Rare species vulnerable to fluctuations or enemies.

More fundamentally: as new species form, old ones must go extinct (unless species numbers increase indefinitely, which geology denies).

Who goes extinct? Common species vary more, improve faster, beat rarer species. Competition is fiercest between closest relatives (varieties, species of same genus). So, new, improving forms will press hardest on their nearest kin and tend to exterminate them. (Same process seen in domestic breeds: improved types replace older ones).

Divergence of Character: Why Life Branches Out

Why do species become clearly distinct, differing more than varieties do? This Principle of Divergence is key.

Problem: Varieties differ less than species. How do small differences become large ones? Chance accumulation isn't enough.

Analogy (Domestication): Breeders create distinct breeds (cattle, pigeons) by selecting for extremes, not average. One selects longer beaks, another shorter; breeds diverge. Nations select swifter vs. stronger horses -> breeds diverge. Intermediate forms aren't selected, disappear. Divergence increases differences.

How it Works in Nature: The more descendants of a species become diversified (in structure, habits, constitution), the more different ecological niches ("places in the polity of nature") they can exploit. This allows more individuals to survive and multiply.

Examples: Carnivore descendants specializing on new prey or habitats. Grass experiment: mixed genera yield more than one species; mixed varieties yield more than one variety. Implies most distinct grass varieties would succeed best, eventually becoming species.

Evidence: Small areas with intense competition have high diversity (turf plot example). Naturalized plants are diverse, often from new genera, showing diversification is advantageous.

Like Division of Labor: Advantage is like physiological division of labor in a body (specialized organs work better). More diversified life forms exploit environment more efficiently -> support more individuals. (Poorly diversified Australian marsupials couldn't compete well with specialized placental mammals).

How Natural Selection Causes Divergence (Using Diagram)

Imagine the diagram from the previous chapter (letters A-L are related species, A is common/varying).

Variation & Selection: A produces diverse variations (dotted lines).

Divergence Favored: Natural selection, guided by the benefit of diversification, tends to preserve the most different (divergent) variations.

Variety Formation: Over time (e.g., 1000 generations), distinct varieties form (a1, m1).

Continued Divergence: These varieties also vary, inherit advantages, and their most divergent offspring are selected (a1 -> a2; m1 -> m2, s2). They become more different from parent A and each other.

Multiplication & Extinction: Process continues. Descendants multiply and diverge. Later, improved branches often replace earlier, less improved ones (extinction of intermediate forms). Original parent species (A) also likely goes extinct.

Species & Genus Formation: After enough time (e.g., 14,000 generations), descendants of A (a14-m14) are very different, possibly forming distinct species or even genera/sub-genera, clustered based on how long ago they branched off. If another species (I) also diverges, its descendants (n14-z14) form another distinct group. Since intermediate species linking A and I likely went extinct, the two groups look like very distinct genera or families.

Classification Explained: This branching process, driven by natural selection favoring divergence and causing extinction of intermediates, directly explains the hierarchical grouping of all life (species in genera, genera in families, etc.). It's not arbitrary, but reflects genealogy.

The Tree of Life

This branching pattern of descent explains why classification looks like a great tree.

Green twigs = living species.

Past years' growth = extinct species.

Branches competing = species/groups competing, causing extinction.

Main limbs from earlier twigs = major groups originating from ancient species.

Few ancient species have living descendants (like few original bush twigs become main branches).

Dead branches = extinct orders, families, genera known only as fossils.

Occasional low, surviving branch = rare "living fossils" connecting groups (Platypus, Lungfish), saved by protected environments.

Conclusion: The Tree of Life, with its dead branches filling the Earth's crust and living branches covering the surface, represents classification and evolution through generation and natural selection.

Overall Trend: Advance in Organization?

Natural selection leads to improvement relative to conditions. This inevitably leads to gradual advancement in organization for most beings.

But defining "advance" is hard (see earlier discussion).

Using Von Baer's standard (differentiation/specialization), selection favors it as it's advantageous.

Low forms preserved: Because high organization is not always advantageous, especially in simple conditions. Selection leaves them unimproved. Geology confirms ancient simple forms persist.

Hierarchy preserved: Higher forms don't always compete with or exterminate lower forms (mammals vs fish; advanced fish vs. simple lancelet). Different niches allow coexistence. Lowly forms may persist in protected areas.

Reasons for low forms: Favorable variations may not have arisen; insufficient time; some regression; mainly, high organization not useful/harmful in simple conditions.

Convergence of Character? Unlikely

Could descendants of different ancestors evolve to become almost identical ("converge")? Conceivable, but extremely unlikely for the whole organism due to the immense complexity of factors shaping form (variation causes, selection pressures, inheritance). If it happened, we'd find identical forms reappearing in distant geological times, which we don't.

Limit on the Number of Species?

Does divergence lead to unlimited species?

Argument for No Limit: Organic conditions become more complex as species increase, creating more niches -> potentially infinite diversification.

Checks on Increase:

Total amount of life an area supports is limited.

Many species -> fewer individuals per species -> higher extinction risk from fluctuations. (Extinction fast, production slow).

Rare species vary less -> slower production of new forms.

Rare species suffer inbreeding.

Most Important? Dominant species spread widely, supplanting many others globally, checking indefinite increase.

Conclusion: These factors together limit the number of species.

Chapter Summary: Natural Selection Explained

Variation is universal; struggle for life is universal (due to geometric increase).

If useful variations occur (likely in complex world), individuals possessing them survive best and reproduce (Natural Selection / Survival of the Fittest).

Leads to adaptation/improvement, generally advancing organization, but simple forms persist if suited.

Acts at all ages; aided by Sexual Selection (male rivalry/charms).

Evidence follows in later chapters.

Entails Extinction (geologically proven).

Leads to Divergence of Character (more diversity supports more life); small differences magnify into species/genus differences.

Common, widespread species (in larger genera) vary most, transmit success.

Divergence + Extinction explain Classification (groups within groups reflect branching descent – the Tree of Life).

Understanding How Life Varies: Laws of Variation

So far, I've sometimes talked about variations – the differences we see between individuals – as if they just happen by "chance." This is, of course, not scientifically accurate. It's just a way of admitting we don't know the exact cause of every single variation.

Some scientists think that producing small differences (variations) is just as much a natural job of the reproductive system as making offspring similar to their parents.

However, consider these points:

Variations and "monstrosities" (major deviations) happen much more often under human domestication than in the wild.

Species that live over wider geographical areas tend to vary more than species living in small, restricted areas.

These observations suggest that variability is generally linked to the conditions of life a species experiences over several generations. In the first chapter, I suggested that changing conditions affect organisms in two ways:

Directly: Acting on the body (either specific parts or the whole organism).

Indirectly: Affecting the reproductive system.

Remember, there are always two factors: the nature of the organism itself (which seems most important) and the nature of the conditions. Direct action by conditions can lead to:

Definite results: Where all or most individuals change in the same way (if the organism's nature allows it).

Indefinite results: Where the organism seems to become "plastic," leading to lots of unpredictable, fluctuating variability.

Direct Effects of Conditions: How Much Impact?

It's very hard to figure out exactly how much effect conditions like climate or food have had directly on organisms in a definite way. Over long periods, the effects might be greater than we can easily prove.

However, we can be confident that the countless amazing and complex adaptations we see in nature – how different parts of an organism fit together, how organisms fit their environment, and how different species interact – cannot be explained only by the direct action of conditions.

Here are some possible examples where conditions might have caused a slight, definite effect:

Brighter shell colors in warmer, shallower water (according to E. Forbes, though not always true).

Brighter bird colors under clearer skies (according to Mr. Gould).

Colors of insects affected by living near the sea (according to Wollaston).

Plants near the seashore developing somewhat fleshy leaves (according to Moquin-Tandon).
These cases are interesting because the variations resemble traits found in species that naturally live only in those specific conditions.

Separating Direct Effects from Natural Selection

When a variation is even slightly useful, it's hard to tell how much change is due to natural selection accumulating small advantages over time, and how much is due to the direct, definite action of the environment.

Example: Thicker Fur: Animals living further north have thicker, better fur. How much is due to selection favoring the warmest individuals over generations? How much is the cold climate directly affecting the fur? (Climate does seem to have some direct effect on the hair of our domestic animals).

Internal Tendencies Matter More Than Conditions?

Sometimes:

Similar varieties arise from the same species under very different conditions.

Different varieties arise under apparently the same conditions.

Many species remain completely unchanged even when living in very different climates.

These points make me think that the direct action of conditions is less important than some internal "tendency to vary," caused by factors we don't yet understand.

Conditions Also "Include" Selection

In a way, the conditions of life do include natural selection, because the conditions ultimately determine which variations survive and which don't. But when we look at human selection, we see two distinct parts:

Variability is somehow triggered or "excited."

The human breeder's will accumulates these variations in specific directions.
Natural selection (the survival of the fittest) is nature's equivalent of the breeder's will accumulating useful changes.

Effects of Use and Disuse (Combined with Natural Selection)

Based on domestic animals (Chapter 1), there's little doubt that:

Use strengthens and enlarges parts.

Disuse weakens and shrinks parts.

These changes can be inherited.

In nature, it's hard to measure these effects because we don't know the original ancestor for comparison. But many animal structures seem best explained by disuse over long periods.

Flightless Birds: Professor Owen called a bird that can't fly a great anomaly, yet several exist.

The logger-headed duck flaps on water like a domestic duck; interestingly, the young can fly, but adults lose the ability.

Large ground birds rarely fly except to escape danger. Birds on oceanic islands with no predators often became nearly wingless, likely through disuse.

The ostrich lives on continents with predators but defends itself by kicking. Its ancestor might have been like a bustard (a flying bird). As ostriches got larger and heavier over generations, legs were used more, wings less, until flight became impossible.

Missing Feet in Beetles: Kirby noticed male dung beetles often have their front feet (tarsi) broken off. One species (Onites apelles) loses them so regularly it was described as naturally lacking them. In others, they are present but tiny (rudimentary). In the sacred scarab beetle (Ateuchus), they are completely gone.

Could this be inherited mutilation? Maybe (Brown-Séquard's experiments on guinea pigs show inherited effects of operations are possible), but it seems safer to assume it's due to long-continued disuse. Since dung beetles often lose their front feet early in life, these feet probably aren't very important or much used.

Wingless Beetles of Madeira: Sometimes we might blame disuse for things caused mainly by natural selection. Mr. Wollaston found that about 200 out of 550 beetle species on the island of Madeira cannot fly! Many entire groups (genera) unique to the island are all flightless.

Evidence Suggesting Selection: Beetles often get blown out to sea and die. Madeira beetles hide until winds calm down. Wingless beetles are even more common on the smaller, more exposed Desertas islands nearby. Crucially, large groups of beetles that absolutely need wings to survive are almost absent from Madeira.

Conclusion: Wollaston and I believe the winglessness is mainly due to natural selection, probably helped by disuse. For generations, beetles that flew less (due to slightly smaller wings or lazy habits) had a better chance of surviving because they weren't blown out to sea. Active fliers were more likely to be lost.

Flying Insects of Madeira: Interestingly, insects on Madeira that need wings to feed (like certain flower-feeding beetles and butterflies) seem to have wings that are not reduced, perhaps even enlarged (Wollaston suspects). This fits with natural selection. When an insect first arrived, selection would favor either better flying (to fight the wind) or giving up flight entirely, depending on which strategy saved more individuals. (Like shipwrecked sailors: good swimmers benefit from being stronger swimmers; bad swimmers benefit from not being able to swim at all and clinging to the wreck).

Eyes of Burrowing Animals: Moles and some rodents that burrow have tiny, rudimentary eyes, sometimes completely covered by skin and fur. This is likely due to disuse, maybe helped by natural selection. The tuco-tuco (a South American rodent) lives even more underground than moles. A Spaniard told me they were often blind; one I kept alive certainly was (due to inflammation). Since eye inflammation is harmful, and eyes aren't needed underground, reducing eye size and covering them could be advantageous. If so, natural selection would help the effects of disuse.

Blind Cave Animals: Animals from very different groups living in deep caves (like in Carniola, Europe, and Kentucky, USA) are blind.

Some cave crabs still have the stalk where the eye used to be, but the eye is gone.

Since useless eyes probably aren't harmful in darkness, the loss is likely due to disuse.

(One cave rat found near the entrance still had large eyes and regained some vision after exposure to light).

Affinities: If these animals were separately created for caves, we might expect American and European cave faunas to be very similar. They aren't. Instead, cave animals tend to be related to the surface animals living in the same region (Europe or America). This makes sense if ordinary animals gradually migrated into caves over generations, losing their eyes through disuse, while selection modified other parts (like making antennae longer for sensing). It's hard to explain these relationships if they were independent creations. The similarities between some Old and New World cave species reflect the general relationship between the faunas of the two continents. (Some blind insects are found outside caves too, suggesting vision loss sometimes happens first). It's not surprising some cave animals are very unusual ("wrecks of ancient life"), as they face less intense competition in their isolated, dark homes.

Acclimatisation: Getting Used to Climate

Habit is Inherited: Plants show inherited timing (flowering, sleep) and needs (rain for germination). This relates to acclimatisation.

Evolution Implies Acclimatisation: Since related species often live in very different climates (hot vs. cold), the idea of descent from a common ancestor implies that species must be able to adapt to new climates over time.

Species are Adapted, But Maybe Not Perfectly: Each species is adapted to its home climate. But we often overestimate how strict this adaptation is. We can't always predict if an imported species will survive our climate, yet many imports do fine. Competition with other species often limits range as much as or more than climate does.

Evidence of Natural Acclimatisation: A few cases show plants naturally getting used to different temperatures (Hooker's Himalayan plants; Thwaites' Ceylon plants; Watson's Azores plants). Animals have also extended their ranges into different climates within historical times. (We don't know for sure if they were strictly adapted before, or became specially adapted later).

Domestic Animal Flexibility: Early humans likely chose animals for domestication because they were useful and bred easily, not because they could handle diverse climates. The fact that domestic animals do tolerate and breed in vastly different climates suggests many wild animals might also have this flexibility. (Though mixed ancestry could contribute). Rats and mice have spread globally with humans, showing wide tolerance.

Flexibility is Key: Maybe adaptation to a specific climate is just a trait easily added onto a general, widespread flexibility of constitution. This view explains why humans, domestic animals, and even extinct elephants/rhinos could handle very different climates than their relatives do today – it's common flexibility brought into play.

What Causes Acclimatisation? It's unclear how much is due to:

Habit/Custom: Seems to play a role (analogy, ancient farming advice about moving animals). Domestic breeds likely adapted through habit, not just human selection for climate tolerance.

Natural Selection: Would inevitably favor individuals born with constitutions better suited to the local climate. Plant varieties are known to differ in climate tolerance (US fruit trees). (The Jerusalem artichoke argument against acclimatisation is flawed because it's not seed-propagated, so no new variation/selection occurs. The kidney bean argument needs a proper multi-generational selection experiment).

Overall: Habit, use, and disuse have likely played a significant role in modifying constitution and structure. But these effects are often combined with, and sometimes overridden by, natural selection acting on innate differences.

Correlated Variation: Linked Changes

The whole organism develops as a connected system. When small variations occur in one part and are accumulated by natural selection, other parts often get modified too. This is correlated variation. It's important but poorly understood. We might confuse different things here:

Simple Inheritance: Can sometimes mimic correlation.

Developmental Links: Changes early in life (embryo/larva) naturally affect the adult form.

Homologous Parts: Parts that share the same underlying structure (e.g., right and left sides, front and hind limbs, maybe even jaws and limbs) and develop under similar influences tend to vary in similar ways. Selection can modify this tendency (like the one-antlered stag). Homologous parts also tend to stick together (seen in "monstrous" plants, normal fused petals).

Physical Effects: Hard parts might affect the shape of adjacent soft parts (bird pelvis/kidney shape? mother's pelvis/baby's head?). Body shape/swallowing might affect organ position (snakes).

Unknown Bonds: Often, the reason for correlation is obscure. Certain malformations often occur together, others rarely. Strange examples:

White cats + blue eyes + deafness.

Tortoiseshell cat color + female sex.

Pigeon feathered feet + webbed outer toes.

Pigeon hatchling down color + adult plumage color.

Hairlessness + poor teeth in naked Turkish dogs (maybe homology?).

Whales/Edentates (abnormal skin) often have abnormal teeth (but many exceptions).

Flower Example (Outer vs. Inner Florets): In daisies (Compositae) and carrot relatives (Umbelliferae), outer flowers often differ from inner ones (petal shape, aborted reproductive parts, seed shape). Why? Pressure? Nutrient flow? Attracting insects (Sprengel)? Selection might act on petals (attracting insects). But seed differences seem unrelated to benefit, yet are used for major classification! Shows important traits can arise just from laws of variation/correlation, without being useful.

False Correlations

Inheritance: Sometimes parts seem correlated just because an ancestor acquired two independent traits at different times, and both were passed down together to descendants with diverse habits.

How Selection Works: Winged seeds are never in fruits that don't open. Why? Selection can only favor slightly better wings if the seeds can actually get out to be dispersed by wind.

Compensation and Economy of Growth

The Law: Geoffroy and Goethe suggested a "law of compensation" – to develop one part more, nature economizes elsewhere. Seems partly true in domestic animals (hard to get cow that gives much milk and fattens easily; cabbage variety good for leaves or oily seeds, not both; larger fruit when seeds shrink; big head crest vs. small comb in poultry).

In Nature: Less clear if universal. Hard to distinguish true compensation (nutrient withdrawal) from selection reducing one part while enlarging another.

General Principle (Economy): Maybe compensation is part of a broader principle: natural selection constantly tries to economize the organism's resources. If a structure becomes less useful (due to changed habits), reducing it is favored because it saves energy/nutrients.

Example (Parasitic Barnacles): When a barnacle lives protected inside another, it loses its own protective shell (Ibla, Proteolepas). Saving the cost of building a large, now useless structure is advantageous in the struggle for life.

Conclusion: Selection tends to reduce parts that become unnecessary, without necessarily causing another part to enlarge. Conversely, selection can enlarge one organ greatly without requiring reduction elsewhere.

What Kinds of Parts Vary Most?

Multiple Parts: Rule (Is. Geoffroy): Parts repeated many times in an individual (snake vertebrae, many flower stamens) tend to vary in number. They also vary more in structure. Since repetition is often a sign of lower organization (less specialization), this fits the common view that simpler organisms are more variable. Why? If a part has many jobs, selection might not fine-tune its exact shape as much as for a highly specialized part (like a multi-purpose knife vs. a specific tool). Selection only acts for the organism's benefit.

Rudimentary Parts: Generally agreed to be highly variable. Likely because they are useless, so natural selection doesn't act to keep their structure consistent.

Unusually Developed Parts: Rule (Waterhouse, Owen): When a part is developed in a remarkable way in one species compared to its close relatives, that part is often highly variable within the species. (Applies only if development is unusual for that specific group, not like a bat's wing). Rule very strong for secondary sexual characters when unusually developed. (These characters are generally variable anyway). Holds true for hermaphrodite barnacles (Pyrgoma valve example: valves usually constant, but vary wildly in species/individuals of this genus). Holds for birds. Hard to confirm in plants due to their overall variability.

Why? Independent creation offers no explanation. Descent with modification viewpoint: An extraordinarily developed part implies a lot of modification occurred relatively recently. This means there was large and long-continued variability in that part, accumulated by selection. Because it was so variable recently, it's likely still more variable than parts that have been stable for much longer. The struggle between selection (fixing the trait) and variation/reversion continues. Only when the part has been stable for a very long time (like a bat's wing, common to all bats) does it become no more variable than other parts. High variability ("generative variability") is expected when modification was recent and large.

Specific Characters vs. Generic Characters: It's well known that specific characters (traits distinguishing species within a genus) are more variable than generic characters (traits shared by all species in the genus).

Explanation: Generic characters were inherited from a common ancestor before the species split off. They haven't changed much since then, so unlikely to vary now. Specific characters have changed since the split (that's what makes them specific), so they are more likely to still be somewhat variable. (Idea that specific characters are less physiologically important is only partly true). Evidence: When an "important" (usually generic) character does differ between related species, it's often variable within individuals too.

Why? Independent creation gives no reason. Descent view explains it: specific characters varied recently, generic ones haven't.

Secondary Sexual Characters: Highly variable, as noted. Also, species within the same group differ more in these characters than in other parts (compare male birds of different grouse species vs. their females).

Why Less Constant? Accumulated by sexual selection, which is less strict than natural selection (doesn't always involve death, just fewer offspring for losers).

Why So Different Between Species? High variability gives sexual selection lots to work with, leading to large differences.

Secondary Sexual vs. Specific Characters Often Affect the Same Parts: Remarkable fact: traits that differ between sexes often involve the same body parts that differ between related species. Examples: Beetle feet joint numbers; Hymenoptera wing veins; Crustacean antennae/legs.

Meaning: The common ancestor had a variable part. Natural selection used variations to adapt different species to different niches. Sexual selection used variations in the same part to adapt males for competing with other males or attracting females.

Connecting Principles: All these points are linked: Specific > generic variability; high variability of unusually developed parts; variability/diversity of secondary sexual traits; sex/species differences in same parts. They stem from:

Common descent (shared inheritance).

Recently varied parts still prone to vary.

Selection gradually overcoming reversion/variability over time.

Sexual selection being less rigid.

Variations in same parts used for both ordinary (natural selection) and sexual purposes.

Analogous Variation and Reversion: Echoes of the Past

Analogous Variation: Unrelated individuals or species independently develop similar variations.

Domestic Examples: Different pigeon breeds developing reversed head feathers or feathered feet (not found in ancestor). Pouter having extra tail feathers (like Fantail). Swollen stems in turnips/rutabagas. Due to shared ancestry and constitution responding similarly to triggers.

Natural Examples (Walsh): Insects varying similarly under similar conditions ("Equable Variability").

Reversion: Reappearance of ancestral traits lost for generations.

Pigeon Example: Slaty-blue color and specific bars/marks (like wild rock pigeon) reappearing in various breeds, especially crosses. Clearly reversion, not new variation.

Mechanism: Surprising after maybe hundreds of generations. Tendency can linger even with tiny fraction of ancestor's "blood." Hypothesis: Not a sudden jump, but a latent tendency present in each generation, finally expressed under favorable conditions. Transmitting a tendency is no harder than transmitting useless rudimentary organs.

In Nature: Expect species of same genus (common ancestor) to sometimes show analogous variations (resembling each other or varieties). Expect occasional reversions. Hard to distinguish the two without knowing the ancestor. Blue pigeon color inferred as reversion due to suite of marks and appearance in crosses. Varying offspring do sometimes assume characters already present in other group members.

Varieties often "mock" other species in the same genus. Intermediate forms share characters. Constant parts sometimes vary to resemble allied species' parts.

Complex Reversion Case (Horse Genus):

Stripes in Different Species: Asses sometimes have zebra-like leg bars, shoulder stripes. Wild relatives (koulan, hemionus) sometimes show stripes (especially foals). Quagga (body barred) sometimes has leg bars. Domestic horses (many breeds, colors, locations) show spinal, leg, shoulder stripes (often multiple), especially duns. Kattywar breed often heavily striped (plainest in foal). Author's foal showed, then lost, stripes.

Hybrids Show Strong Tendency: Mules (ass x horse) often striped. Ass x Zebra hybrids strongly barred. Quagga x Horse hybrid strongly barred (even pure offspring from same mare later!). Ass x Hemionus hybrid heavily striped (legs, shoulder, face), despite parents having few/no stripes! (Face stripes predicted Kattywar face stripes).

Conclusion: Distinct horse species share tendency to vary towards stripes (especially dun color). Hybrids show it most strongly. Parallel to Pigeons: Blue color/marks reappear on variation/crossing. Hypothesis: Latent tendency from ancient striped ancestor (perhaps zebra-like) expressed sometimes, especially in young or crosses.

Alternative (Rejected): Each species created with tendency to vary like others AND produce hybrids resembling others? Makes creation a "mockery."

Chapter Summary: Laws Governing Variation

Ignorance: We don't know the cause of most specific variations.

Consistency: The same laws seem to produce small variety differences and larger species differences.

Conditions: Cause fluctuating variability, sometimes direct/definite effects (significance unclear).

Habit/Use/Disuse: Potent effects in modifying constitution/structure.

Correlation: Homologous parts vary together, tend to cohere. Hard parts affect soft. Obscure correlations exist. Compensation/economy occur (selection economizes by reducing useless parts).

Variability Patterns:

Multiple parts are variable (less specialized).

Lowly organized beings more variable (less specialized).

Rudimentary organs variable (useless).

Specific characters > generic characters (recently varied).

Unusually developed parts highly variable (recently modified, variability not yet fixed). (Constant if ancient, like bat wing).

Secondary sexual characters highly variable, differ greatly, often affect same parts as specific characters.

Analogous Variation & Reversion: Species with shared ancestry vary similarly, sometimes revert to lost ancestral traits. Adds to diversity.

Ultimate Cause of Major Change: While causes of slight differences are obscure, steady accumulation of beneficial differences by natural selection is likely responsible for all major adaptations related to life habits.

Challenges to the Theory of Evolution by Natural Selection

By now, you've probably thought of many questions and challenges to the ideas presented here. Some of these are very serious, and even today, thinking about them can make me pause. However, I believe most of these difficulties only seem like problems, and the ones that are real don't destroy the theory.

We can group these challenges as follows:

Missing Links: If species gradually changed from other species through tiny steps, why don't we see countless intermediate or "transitional" forms everywhere today? Why does nature look organized into distinct species, instead of being a confusing mess?

Major Transitions & Complex Organs: Is it really possible for an animal like a bat (with wings and flying habits) to have evolved from an ancestor with very different habits and structure? Can natural selection really create both minor things (like a giraffe's tail used as a fly-swatter) and incredibly complex organs like the eye?

Instincts: Can instincts be learned and changed through natural selection? How did bees evolve the instinct to build perfect honeycomb cells, something that seems to require advanced mathematics? (Discussed in a later chapter).

Hybrids: Why are different species usually unable to breed together successfully (or produce sterile offspring like mules), while different varieties of the same species can breed together just fine? (Discussed in a later chapter).

This chapter will tackle the first two points.

Difficulty 1: Why Don't We See Countless Transitional Forms?

Natural Selection Causes Extinction: Remember, natural selection works by preserving helpful changes. In a world full of life, each new, improved form tends to compete with and eventually replace its own less-improved parent form, as well as other competitors. Extinction and natural selection go hand-in-hand. So, the very process of creating a new species usually wipes out the original parent form and all the transitional varieties that linked them.

The Fossil Record Problem: But if countless transitional forms must have existed according to the theory, why don't we find huge numbers of their fossils buried in the Earth? This is a major question we'll explore more in the chapter on the Geological Record. The main answer, I believe, is that the fossil record is far, far less complete than people usually think. It's like a huge museum with collections gathered imperfectly and only once in a long while.

What About Living Intermediate Forms? Okay, fossils aside, shouldn't we find living transitional forms today between closely related species living in the same area?

Consider species that gradually replace each other as you travel north or south. Where they meet, they often overlap slightly, but even there, they usually remain quite distinct from each other.

My theory says these related species came from a common ancestor. Each adapted to its own region and, in the process, wiped out the original parent and the intermediate steps. So, we shouldn't expect to find numerous transitional forms within each species' main territory today (though fossils might exist).

The Intermediate Zone Puzzle: But what about the area between the two species' main territories? This zone often has intermediate conditions. Why don't we find intermediate varieties living there, linking the two species? This puzzled me for a long time.

Possible Explanations for Missing Links Today:

Past Isolation: Areas that are continuous now might have been broken into islands or separate regions in the recent geological past. Distinct species could have formed in these isolated areas. Later, when the areas reconnected, the species might meet without intermediate forms ever having existed between those specific regions. (Isolation likely played a role, but species probably also formed in continuous areas).

Sharp Boundaries Between Species: Species often have large territories where they are common, but become rare quite quickly at the edges and then disappear. The "neutral territory" between two competing species is usually narrow compared to the area each occupies. (This is surprising if only gradual climate changes matter). Why? Because a species' range depends heavily on other species – its competitors, its food, its predators. Since these other species are also distinct, the boundaries tend to become sharp. Also, species at the edge of their range, where numbers are low, are very vulnerable to being wiped out by bad seasons or changes in enemy/prey numbers, making the boundary even sharper.

Intermediate Varieties Are Rarer: If species ranges have sharp boundaries, the same probably applies to varieties. A variety living in the narrow intermediate zone between two main varieties would likely exist in smaller numbers than the main varieties occupying larger areas. (This seems to hold true based on observations of barnacles and information from other naturalists).

Intermediate Varieties Get Exterminated: Rarer forms are always more likely to go extinct by chance. The intermediate variety is especially vulnerable, squeezed between successful forms on both sides. More importantly, during further evolution, the two main varieties (existing in larger numbers over larger areas) have a much better chance of producing further beneficial variations. They will improve faster through natural selection and eventually beat and eliminate the intermediate variety, which exists in smaller numbers and improves more slowly. (Think of sheep breeders: large flock owners on mountains or plains improve their breeds faster than small holders on narrow hills between them; the hill breed gets replaced).

Why Nature Isn't Total Chaos: To sum up, species tend to be well-defined today, not a confusing blend, because:

New varieties form very slowly. Natural selection needs favorable variations and open ecological niches, which appear gradually.

Continuous areas were often broken up in the past, allowing separate species formation.

Intermediate varieties, when they form, usually exist in smaller numbers and are outcompeted and exterminated by the more numerous main varieties evolving on either side.

The very process of natural selection constantly exterminates parent forms and intermediate links over time. Evidence of past links relies on the imperfect fossil record.

Difficulty 2: How Can Major Changes in Habits and Structure Occur?

The Challenge: How can a land animal become aquatic? How could it survive during the transition? How could complex organs evolve?

Transitions in Habits: It's easy to find living animals showing intermediate grades between lifestyles (like the mink, hunting on land and in water). Since each lives successfully, each grade must be viable.

Transitions in Structure (Imagining the Steps):

Flying Squirrels: We see a perfect series today, from squirrels with slightly flattened tails to those with full skin membranes allowing long glides. Each step likely provided an advantage (escape, food gathering, safety). Under changing conditions, selection could easily favor fuller membranes until a perfect "flying" squirrel evolved.

Bats: The flying lemur (Galeopithecus) has a very wide membrane including limbs and fingers. While we don't see living links connecting it to other insectivores, we can easily imagine such links existed and evolved like the squirrels'. We can further imagine selection lengthening the membrane and fingers, eventually turning it into a bat. (Some bats might show traces of a gliding past).

Bird Wings: Consider the diverse uses of wings in birds today – flapping (duck), fins/legs (penguin), sails (ostrich), useless (kiwi). If some of these went extinct, we might never guess such forms existed. This shows that many transitional possibilities exist, even if they don't represent the actual steps to perfect flight (some might be due to disuse).

Flying Fish: These fish glide far using fins. It's conceivable selection could turn them into truly winged animals. If that happened, who would guess their ancestors used early "wings" just to escape predators in the ocean?

Why Transitional Fossils Are Rare: Early transitional forms are usually replaced by their more perfected successors. Also, forms transitioning between very different lifestyles were likely not numerous or diverse initially. Only once an organ was highly developed would many variations exploiting it arise. So, transitional fossils are less likely to be found than fossils of fully developed forms.

Changes and Diversity in Habits Within a Single Species

Natural selection could easily adapt an animal's structure if its habits change, or specialize it for one of its diverse habits. (Did habits change first, or structure? Probably often happened together).

Changed Habits: Many British insects now feed on non-native plants or artificial things.

Diverse Habits: A South American flycatcher hovers like a kestrel and dives for fish like a kingfisher. The European great titmouse climbs like a creeper, kills birds like a shrike, and hammers seeds like a nuthatch. The North American black bear swims for hours catching water insects like a whale.

Species With Habits Different From Their Relatives

Since individuals sometimes adopt unusual habits, new species with strange habits might occasionally arise.

Woodpeckers: Perfectly adapted for climbing and drilling bark. Yet, some North American species eat fruit or chase insects in the air. A woodpecker on the treeless plains of La Plata (Colaptes campestris) has the basic woodpecker structure but nests in holes in banks! (Though sometimes uses trees elsewhere). Another drills wood to store acorns.

Petrels: Normally ocean-flying birds. But Puffinuria in Tierra del Fuego acts exactly like an auk or grebe (diving, swimming), with profoundly modified structure.

Water Ouzel: Looks like a thrush, but dives for food, using wings underwater.

Aquatic Insect: Most Hymenoptera (wasps, bees, ants) are land-based. But Proctotrupes dives and swims underwater using its wings, without any obvious structural changes.

Structure Not Always Matching Habits

If each creature was created perfectly for its purpose, it's surprising when structure and habits don't match.

Webbed feet are for swimming, yet upland geese rarely swim. Frigate birds have fully webbed feet but rarely land on water.

Grebes and coots are highly aquatic but only have flaps on their toes.

Long toes of wading birds are for swamps, yet the related water-hen is aquatic, and the landrail lives in meadows.

Conclusion: In these cases, habits have changed, but structure hasn't fully caught up (or just started to, like the frigate bird's feet). The upland goose's webbed feet are almost useless (rudimentary in function).

Evolutionary View: This isn't surprising if life is a struggle and organisms try to survive wherever possible. If a slight change in habit or structure gives an advantage, an organism might invade a new niche, even one very different from its relatives'. Hence, land geese with webbed feet, meadow birds with wading toes, diving thrushes, etc.

Difficulty 3: Organs of Extreme Perfection (e.g., The Eye)

The Challenge: It seems "absurd in the highest degree" to think an organ as complex and perfect as the eye could be formed by natural selection. (Like thinking the world turned round the sun seemed absurd initially).

The Argument: Reason tells us: if we can show many gradations from a simple to a complex eye exist, and each step is useful to its owner (which is true); AND if eyes vary and variations are inherited (also true); AND if these variations could be useful under changing conditions; THEN the difficulty, though hard for our imagination, shouldn't destroy the theory. (Origin of light sensitivity itself is beyond scope, like origin of life. But simple organisms detect light without nerves, so nerve development seems possible).

Finding Gradations: We can't see direct ancestors. We must look at related living species (collateral descendants) to see possible steps.

Simplest Eye: Optic nerve + pigment + skin (no lens). Detects light/dark only. (Even simpler: pigment spots without nerves?).

Next Step: Starfish have jelly-filled depressions concentrating light – first step towards forming an image.

Arthropods (Insects, Crustaceans, etc.): Show huge range from simple pigment-coated nerves to complex compound eyes with true lenses and modified nerve fibres.

Conclusion for Eye: Given the wide range of eye structures in lower animals, plus countless extinct forms, it's believable that natural selection could convert a simple light-detecting spot into a complex eye like an insect's or even an eagle's. We don't know all the steps for the eagle's eye, but if the theory explains so much else, we should accept it's possible.

Objection (Simultaneous Changes): Modifying the eye requires many parts changing together perfectly. Response: Changes don't need to be simultaneous if they are tiny and gradual. Different kinds of modifications can achieve the same result (like fixing focus by changing lens curvature or density). Features like the iris or eye muscles are improvements that could be added later.

Vertebrate Eye: Also shows grades, starting from a very simple eye in the lancelet (like a pigment-lined sack with a nerve). Fish and reptiles show many steps (Owen). Human eye lens develops from simple skin cells in the embryo (Virchow).

Reason vs. Imagination: To accept eye evolution, reason must overcome imagination. The difficulty is understandable.

Eye vs. Telescope Analogy (Revisited): Inferring eye created like telescope (by intelligence) is presumptuous. Better analogy: Imagine layers of transparent tissue and fluid over a nerve. Suppose layers constantly change slightly in density, thickness, shape, position. A power (natural selection) constantly watches, preserving any change, anywhere, that makes the image slightly clearer under any circumstance. Multiply each new state by millions (generation); preserve until better one arises; destroy old ones. Over millions of years, on millions of individuals, could a living optical instrument far superior to glass be formed? Yes.

Could Any Organ Be Impossible to Form Gradually?

If any complex organ exists that could not possibly have formed by many small, successive steps, the theory fails. I can find no such case.

We often don't know transitional grades (due to extinction, especially around isolated species or ancient organs common to whole classes).

Be cautious assuming gradations impossible.

Possible Modes of Transition

Organ Does Multiple Functions: Lower animals often have one organ doing several jobs (dragonfly gut; Hydra surfaces). Selection could specialize part or all of the organ for just one function, gradually changing its nature. (Plants with different flower types changing suddenly is possible, but likely differentiated gradually first).

Two Organs Do Same Function: One organ can be perfected while aided by another, which is then modified for a different purpose or lost.

Swimbladder -> Lung: Fish breathe water (gills) and air (swimbladder). Swimbladder is homologous to lungs. Shows organ for flotation converted to respiration. Explains why our food pipe crosses our windpipe. Embryo shows gill remnants. Lost gills could have been modified (like insect wings maybe from tracheae).

Barnacle Gills: Pedunculated barnacles use small skin folds (frena) to hold eggs; whole body respires. Sessile barnacles lack frena but have large gills in same spot. Gills clearly homologous to frena. Small folds aiding slightly in respiration were likely converted by selection into main breathing organs.

Change in Reproductive Timing (Cope): Some animals reproduce before fully mature. If this became standard, adult stage might be lost -> species character changes. Some animals change throughout life after maturity (skulls, horns, plumes, teeth, crustacean parts). If reproduction delayed -> adult character modified; earlier stages might be lost. Unsure how often this happens, but if so, differences likely acquired gradually first.

Specific Difficult Cases

Neuter Insects: Workers often different from males/fertile females. (Discussed next chapter).

Electric Organs in Fish: How did these evolve? Use is partly unknown (defense/prey for some; Ray organ very weak). Relation to muscle structure/discharge known, but steps unclear. Problem: Found in ~12 distantly related fish. If inherited, fish should be related. Geology doesn't support widespread loss. Solution: Organs are in different places, differ in structure/nerve supply -> not homologous, only analogous (similar function). Arose independently in each group. Difficulty reduced, but still significant for each case.

Luminous Organs in Insects: Similar problem: few species, different families, different body parts. Parallel difficulty.

Other Analogous Organs: Pollen masses in distant Orchis/Asclepias. Similar function, appearance, but not homologous structure.

Rule: Similar organs in distant groups always show fundamental structural differences.

Cuttlefish vs. Vertebrate Eye: Seem similar, but structure fundamentally different (lens, retina, muscles). Not homologous. Selection produced similar functional result independently.

Fritz Müller's Air-Breathing Crustaceans: Related families share many identical structures (inheritance). BUT air-breathing apparatus (same purpose) differs significantly between them. Makes sense if distinct groups adapted independently, selection working on different variations. Separate creation unintelligible.

Claparède's Parasitic Mites: Hair-claspers in distinct mite families developed independently from different body parts (legs, mouthparts, etc.). Same end, different means.

Infinite Diversity for the Same End

Nature commonly achieves the same goal in vastly different ways (bird wing vs. bat wing vs. insect wings; bivalve hinges; seed dispersal methods). Not "toys," but result of selection working on different variations in organisms with different histories.

Fertilization Example: Wind (simple). Insect attraction via nectar (simple). Leads to countless complex flower adaptations (traps, movements - Coryanthes orchid bucket, Catasetum pollen-shooting).

Why Diversity? When organisms already differ slightly, their variations won't be identical. Selection working for the same goal produces different results. Each structure is sum of inherited past modifications.

Nature Makes No Leaps ("Natura non facit saltum")

Considering transitions, rarely find organ with no known intermediate grades (despite incomplete record).

New organs rarely/never appear suddenly created. Old saying "Nature makes no leaps" generally true for evolution. (Milne Edwards: prodigal in variety, niggard in innovation).

Why variety but little novelty? Why graduated steps? Creation theory offers no answer.

Natural Selection Explains: Acts only by small successive variations -> cannot leap -> must use short, slow steps.

Difficulty 4: Organs of Little Importance

How can selection form parts seemingly unimportant?

Our Ignorance: We can't judge true importance. "Trifling" traits (fruit down, animal color) can be selected if linked to survival (constitution, insect attacks). Giraffe tail (fly-flapper) seems trivial, but resisting insects vital in some regions.

Past Importance: Organ unimportant now might have been vital to ancestor, transmitted since then (e.g., tail from aquatic ancestor used by land animals).

Other Causes: Don't forget direct conditions, spontaneous variation, reversion, growth laws (correlation, etc.), sexual selection creating traits used indirectly later.

Mistaken Importance: We might wrongly assume adaptation. Green woodpecker color probably sexual selection, not camouflage. Palm hooks maybe defense first, climbing later. Vulture head skin? Maybe putrid matter effect, not just adaptation (male turkey head also naked). Skull sutures aid birth, but exist in egg-layers -> likely growth law, utilized later.

Can't Explain Domestic Breed Differences: We are ignorant of causes of variation even in domestic breeds (climate, food, exercise, correlations, unknown factors all play role). If we can't fully explain them, shouldn't overstress ignorance about species differences.

Objection: Non-Useful Structures (Beauty, Variety)

Some argue structures exist just for beauty (man's/Creator's) or variety. Fatal to theory if true.

Response: Admit many structures not now useful. Doesn't mean solely for beauty/variety. Other causes (conditions, growth laws) have effects. Inheritance is key: much structure inherited, related to past needs, not present habits (upland goose feet; homologous bones in arm/leg/wing/flipper). These useful to ancestors.

Beauty for Man? Sense of beauty not innate/universal (different races). Less beauty expected before man? Were fossil shells made for cabinets? Microscopic diatoms? Flower beauty linked to insect attraction (wind-pollinated drab). Fruit beauty guides dispersers (birds/beasts).

Beauty for Beauty's Sake: Yes, in male animals (birds, butterflies). Caused by sexual selection (females prefer beautiful males), not man's delight. Bird song too. Implies shared taste for beauty/sound in animals. Female beauty results from transmission from males. Origin of aesthetic sense obscure (like tastes/smells).

No Altruistic Structures: Natural selection cannot make parts solely for another species' good. Species exploit each other, but modifications benefit possessor. Can't find valid case otherwise. Rattlesnake rattle likely warns enemies, not prey.

No Malicious Structures: Selection won't make parts solely for injury/pain to possessor (Paley). Overall balance advantageous. If part becomes injurious -> modified or extinct.

Perfection is Relative: Selection makes beings only as perfect as (or slightly better than) local competitors. Standard is relative (NZ natives yield to Europeans). Absolute perfection not goal or result. Eye isn't optically perfect (Helmholtz). Nature has "contradictions." Reason sees imperfections (bee sting killing bee). Understandable if sting evolved from borer; useful for community despite individual loss. Hive drones useful only for mating? Queen bee killing daughters good for community? Fir tree pollen clouds inefficient?

Summary of Difficulty Discussion: Many difficulties serious, but exploring them illuminates facts obscure under creation view. Species distinct now because selection slow, exterminates intermediates, past isolation occurred. Transitions in habits/structure possible. Organs of perfection explainable by gradual steps. Similar organs in distant forms analogous, not homologous. Diverse means achieve same end. Organs of small importance explainable (past use, growth laws, etc.). Perfection relative.

Two Great Laws Explained:

Unity of Type: Fundamental shared structure within class (independent of habits). Explained by common descent.

Conditions of Existence: (Cuvier). Embraced by natural selection, which adapts beings to conditions (past/present), aided by use/disuse, direct effects, growth laws.

Conclusion: Conditions of Existence is higher law, including Unity of Type through inherited adaptations.

More Questions About the Theory

Okay, here is a simplified rewrite of that chapter addressing various objections and difficulties with the theory of natural selection.

This chapter looks at various other objections raised against my theory of evolution by natural selection. Discussing these might help clarify some earlier points. It’s impossible to cover every objection, especially those made by writers who haven’t fully grasped the subject.

Common Misunderstandings

Are All Beings Imperfect? A German scientist claimed my theory relies on all living things being imperfect. That's not what I said. My point is that organisms are not as perfectly adapted to their conditions as they could be. This is proven by the fact that introduced species often outcompete native ones. Also, if conditions change (which they always do), organisms must also change to remain adapted.

Does Selection Favor Longer Life? A critic argued that since long life is advantageous, natural selection should always lead to descendants living longer than ancestors. But imagine a plant living two years that spreads to a cold climate. It might die each winter but survive year after year using seeds. Longevity seems related more to an organism's complexity, reproductive effort, and activity level – all factors likely shaped by natural selection itself (as Mr. E. Ray Lankester discussed).

Egyptian Animals Haven't Changed? Some argue that because Egyptian animals look the same as 3,000-4,000 years ago, no species have changed anywhere. As Mr. G. H. Lewes pointed out, this argument proves too much. Ancient domestic animals from Egyptian tombs also look identical to modern breeds, yet we know they were produced by modifying wild ancestors. A stronger case would be animals unchanged since the last Ice Age – they endured huge climate changes and migrations. Egypt, in contrast, has had relatively stable conditions recently. Lack of change doesn't disprove natural selection, which only acts when beneficial variations arise and circumstances allow them to be preserved. It mainly challenges ideas about an automatic, internal drive for development.

Further Objections and Replies

Objection: How Can Varieties Live Alongside Parent Species? (Bronn)

Answer: They could coexist if they adapt to slightly different lifestyles or conditions (like living at different altitudes or in wet vs. dry areas). Also, for animals that move around and breed freely, varieties are usually found in geographically separate regions. (This doesn't apply to highly variable "polymorphic" species or temporary variations like size changes or albinism).

Objection: Species Differ in Many Parts, Not Just One. How Can Selection Change Many Things at Once? (Bronn)

Answer: There's no need to assume all parts change simultaneously. Major adaptations can be built up step-by-step: a slight, useful variation in one part is selected, then later, a slight variation in another part. Because these changes are inherited together, they look like they happened at the same time. Domestic breeds show this: racehorses and draft horses differ completely, but the changes happened gradually, part by part. Even when humans select strongly for just one trait (like a flower or fruit in plants), other parts of the plant almost always change slightly too, due to correlated growth (parts being linked) and other variation causes.

Objection: Many Traits Seem Useless. How Could Selection Create Them? (Bronn, Broca, Nägeli)

This is a serious point. Critics list examples like ear/tail length in mice/hares, complex tooth enamel folds, and many plant features (cell patterns, leaf arrangements, flower part numbers, ovule positions, seed shapes) that seem unimportant for survival. Nägeli argued families of plants differ mainly in these "useless" traits and believed in an innate drive towards progressive development.

Response 1: We Are Ignorant of True Usefulness. We should be extremely cautious about declaring any structure useless. We often don't know the full life story or needs of a species. Many complex organs whose uses were unknown have recently been found to be important (like the nerves in a mouse's ears suggesting they are used for touch; the tail in some mice is used for grasping).

Response 2: Laws of Growth Cause Changes. When one part changes (due to selection or other causes), other parts often change too because of how organisms grow and develop. These "laws of growth" include:

Changes in nutrient flow.

Pressure between parts during growth.

Effects of early development on later parts.

Mysterious correlations we don't understand.

Response 3: Direct Effects & Spontaneous Variation. We must also consider direct effects of the environment and "spontaneous" variations (like a moss rose appearing on a common rose bush) that seem largely independent of conditions. Every variation must have a cause; if that unknown cause acted consistently, it might modify the whole species uniformly.

(Self-Correction): I probably underestimated the importance of spontaneous variability in earlier editions. However, it's impossible to believe that the countless perfect adaptations we see are due only to this, any more than a racehorse's form arose without selection.

Examples (Plants): Many orchid structures once seemed pointless morphological details, but are now known to be vital adaptations for insect pollination, likely shaped by selection. Different lengths of stamens/pistils in dimorphic/trimorphic flowers are now known to be crucial for proper fertilization. Ovule position (upright vs. hanging) seems minor, but might affect which ovules get fertilized depending on how pollen tubes grow (Hooker). Closed, self-pollinating flowers often show major structural changes (reduced petals, stamens, etc.). Selection likely favored flower closure and reduced pollen, but the specific structural changes probably result from laws of growth and disuse during that process.

Growth Laws Example (Position Effects): Parts can differ based on their position on the same plant. Leaf angles on chestnut/fir branches. Central vs. side flowers having different numbers of parts (Rue, Adoxa). Outer vs. central flowers differing in petal size, reproductive organ development, seed shape/color/features (Compositae, Umbelliferae). Different fruit types on the same plant (Fumaria). These position-dependent changes (except maybe large ray flowers attracting insects) likely aren't due to selection but to growth laws and interactions between parts.

Unexplained Variations: Sometimes important structural variations occur randomly on some flowers or plants but not others, under the same conditions. Use unknown, cause unknown (not just position). Examples: Flowers varying in part numbers (poppy); petal folding varying (Mimulus); ovary number varying (Zanthoxylon); capsule structure varying (Helianthemum); placentation varying (Saponaria); two distinct forms on one Gomphia bush. Conclusion: These changes seem unimportant to the plant's welfare (their variability suggests this). They don't show "progression"; closed flowers might even show regression. They likely arise from unknown causes; if the cause acted uniformly, the whole species might change.

Fate of Unimportant Characters: Natural selection doesn't accumulate slight variations in useless traits. (Useless parts, like rudimentary organs, become variable because selection stops regulating them). However, unimportant modifications caused by growth laws or other factors can be inherited stably. Hair on mammals, feathers on birds, scales on reptiles might be examples – widely transmitted even if not initially vital. Result: Systematists often rank characters common to large groups as highly important, assuming they are vital. But maybe many important morphological characters (leaf arrangement, flower parts) first arose as unimportant fluctuations, became fixed by internal factors or crossing, not by selection. Paradox: Characters of slight importance to the species are often the most important for classification (explained later by common descent).

Progressive Development: No evidence for an innate drive. But progress does result from natural selection constantly favoring better adaptations, often leading to increased specialization (differentiation) of parts, which is the best definition of higher organization.

Reply to Mr. Mivart's Collected Objections

A recent book by Mr. St. George Mivart skillfully gathers objections against natural selection (made by myself and others). It's a strong presentation, but doesn't include the counter-arguments, requiring effort from the reader to weigh both sides.

Mivart overlooks the effects of use and disuse (which I've always stressed).

He assumes I attribute nothing to variation independent of selection (incorrect; my Variation book details many such cases).

After reading Mivart, I feel more convinced of my theory's general truth (allowing for errors in such complex subjects).

Mivart's Key Objection: Incipient Stages of Useful Structures: He argues natural selection can't explain the very beginning stages of useful structures, before they are fully functional. This relates to gradations and function changes discussed last chapter. Let's look at his examples:

Giraffe: Its whole body is adapted for Browse high branches (advantage during food shortages – contrast with Niata cattle that perish). Selection would favor individuals that could reach even slightly higher. Slight variations in limb/neck length occur. Selection preserves those with longer parts; they interbreed; inherited effects of use also help coordinate the parts. Seems almost certain an ordinary hoofed animal could become a giraffe this way.

Mivart's Objection 1 (Food Supply): Larger size needs more food, a disadvantage in scarcity? Counter: Giraffes thrive in Africa alongside other large antelope. Reaching untouched food is an advantage. Bulk protects from predators (except lions); tall neck acts as watchtower and weapon. Survival depends on multiple factors.

Mivart's Objection 2 (Why Only Giraffe?): Why haven't other hoofed animals gotten long necks or trunks? Counter: In Africa, competition for high Browse is between giraffes. Other animals browse lower (like sheep vs. cattle in a field – no advantage for sheep to get longer necks). Only the highest browser could benefit from further neck elongation. Why not elsewhere? Unanswerable specifically (like asking why historical events differ). We don't know all factors. Maybe conditions didn't favor large size; maybe needed co-adaptations didn't vary correctly; maybe other pressures (predators, parasites) prevented selection for Browse height; maybe conditions weren't stable long enough.

Other Cases of Non-Development: Why no flight for ostrich? (Needs enormous food supply). Why no land mammals evolved from seals/bats on oceanic islands? (No suitable prey for land carnivore from seal; bats face competition from existing reptiles/birds, ill-suited for ground locomotion needed first). Transitions require peculiar conditions and serviceable intermediate steps.

Mental Powers: Why aren't apes as smart as humans? Why haven't all animals become highly intelligent? Many possible reasons (conjectural). Can't even explain why one human tribe becomes more civilized than another.

Mimicry (Insect Camouflage): Insects resemble leaves, twigs, etc., often wonderfully well. Mivart: How can indefinite tiny variations build up such resemblance? They'd cancel out. Counter: Assumes an initial rude, accidental resemblance to a common object (likely given infinite objects and insect forms). Selection preserves variations making resemblance closer, eliminates others. Objection only valid if relying on random fluctuation alone, not selection guiding it. Perfection refined because predators have sharp eyes; every improvement helps escape. Variations likely occur in traits already differing between related species (surface texture, color).

Whale Baleen: Huge plates for filtering tiny prey. How did it begin? Counter: Imagine early whale ancestor with mouth like a duck's beak (lamellae). Ducks sift water. Gradations exist in ducks: Goose (tears grass, coarse knobs) -> Egyptian Goose (graze/sift) -> Common Duck (sift/crush) -> Shoveller (highly efficient sifter). Lamellae get finer, more numerous, project more. Goose beak could also modify into fish-catching teeth (Merganser). Plausible early whale had horny points -> selected for sifting -> like Egyptian Goose -> Duck -> Shoveller -> giant baleen plates. Each step useful, function slowly changed. Habit/use played little role here.

Flatfish Eye Migration: Eyes move to top side. Mivart: Sudden change unlikely; gradual seems harmful initially. Counter (Malm): Young flatfish are symmetrical but can't stay upright -> fall to one side -> habitually twist lower eye upwards to see -> flexible skull yields -> eye moves slightly. Inherited effects + continued use make it permanent. Mouth also bends down from use (feeding on bottom). Lower side less developed from disuse/no light. Speckled top color = selection. Use/disuse + selection work together.

Prehensile Tails: Mivart reviewer: Impossible first slight grasping tendency useful. Counter: Habit likely sufficient. Young monkeys use tails for support. Harvest mice (non-prehensile tail) use tails to help climb. If more tree-dwelling, tail could become structurally prehensile. (Why not Cercopithecus? Maybe tail more useful for balance).

Mammary Glands: Mivart: Young saved by accidentally sucking drop from enlarged skin gland? Unlikely. Counter: Mammals likely from marsupials -> glands first developed in pouch. Maybe early young nourished by pouch gland secretions (like possibly Hippocampus fish)? Selection favors individuals secreting better fluid -> glands improve, concentrate (breast). Young learning to suck no harder than chick hatching/pecking. Kangaroo larynx adaptation prevents choking milk injection; why lost? Maybe hindered voice or swallowing solids.

Echinoderm Pedicellariæ (Pincers): Mivart: Utility of beginnings? How stalk + jaws evolve together? Counter: Fixed pincers exist (defense?). Gradations exist: 3 arms -> 2 + support -> 2 arms. Origin: modified spines (Agassiz). Gradations from simple granules -> spines -> branching spines -> fixed pedicellariæ -> stalked pedicellariæ. All steps likely useful (defense, grasping).

Polyzoa Avicularia & Vibracula: (Defensive organs). Mivart: Similarity to pedicellariæ = problem (independent origin). Counter: No real structural similarity! Avicularia homologous to zooids (basic units of colony). Vibracula (bristles) also homologous to zooids, likely via avicularium stage (gradations exist). Shows two different organs from one source; all steps likely protective. (Crustacean pincers show clear serviceable gradations).

Orchids (Pollinia): Mivart: Incipient stages explanation insufficient. Counter: Gradations clearly exist and are useful. Pollen threads -> nascent caudicle -> full caudicle attached to viscid mass (modified stigma). Each step aids insect pollination.

Climbing Plants: Mivart questions origins. Counter: Gradations twiners -> leaf-climbers -> tendril-bearers clear, each step advantageous. Twining based on existing plant movements (revolving); sensitiveness based on existing sensitivity to touch. Selection enhanced incidental abilities for climbing.

Recap of Transitions: Gradual steps feasible for giraffe, mimicry, baleen, flatfish eye, prehensile tail, mammary glands, pedicellariæ, Polyzoa organs, pollinia, climbing plants. Use/habit important for some, selection for others, often combined.

Why Are Transitions Not Always Made? Can't expect specific answers. General reasons: needed variations didn't occur; other pressures prevented selection; conditions not stable long enough. Transitions complex; not strange they fail sometimes. Belief any beneficial structure would evolve is wrong.

Abrupt Modifications vs. Gradual Evolution:

Mivart and others favor sudden leaps (internal force). Examples: Hipparion -> horse; bird wing development.

Author Rejects: Improbable. Admits changes as large as single known variations possible, but natural variations likely smaller/rarer than domestic ones (many of which are reversions/monstrosities). Sudden natural change needs multiple individuals changing simultaneously (unlikely). Gradualism avoids this (preserves many slightly varying individuals).

Evidence for Gradualism: Closely allied species/genera numerous. Representative species geographically/geologically linked. Fine gradations in structures between allied species. Larger genera having more related/variable species. Specific characters > generic variability. Extraordinary parts more variable. Embryology shows gradual differentiation (wings, legs start similar). Embryo reflects ancestry; abrupt changes should leave trace but don't.

Conclusion: Belief in sudden transformations (internal force, multiple simultaneous changes, complex co-adaptations appearing instantly, no embryonic trace) enters realm of miracle, not science.

Final Thoughts

This chapter addressed various objections. While some are serious, exploring them often reveals how the theory explains facts otherwise obscure under separate creation. Key points reaffirmed: species aren't infinitely variable now; selection is slow; extinction removes links; transitions in habit/structure are possible through intermediate steps; complex organs like the eye can be conceived as forming gradually; organs can change function; similar structures in distant relatives arise independently (analogy, not homology); diverse structures evolve for the same purpose; organs unimportant now might have past uses or arise from growth laws; perfection is relative, not absolute. The theory embraces two great laws: Unity of Type (explained by common descent) and Conditions of Existence (explained by natural selection adapting organisms to their environment over time).

More Questions and Challenges for the Theory

In this chapter, we'll look at several other objections that have been raised against my theory of evolution through natural selection. Talking about these might make some earlier points clearer. It's pointless to discuss all objections, as many come from writers who haven't fully understood the ideas.

Addressing Some Misunderstandings

Are all living things imperfect? A respected German scientist claimed my theory says all organisms are imperfect. That’s not quite right. What I've said is that organisms are not always as perfectly suited to their current conditions as they could potentially be. We see proof of this when foreign species invade a new land and successfully outcompete some of the native inhabitants. Also, even if organisms were perfectly adapted at one time, they wouldn't stay that way if their environment changed, unless they also changed. Everyone agrees that environments and the mix of inhabitants constantly change over time.

Should descendants always live longer? A critic, using math, argued that since long life (longevity) is always an advantage, natural selection must make descendants live longer than their ancestors. But couldn't a two-year plant, or a simple animal, adapt to a cold climate by surviving the winter only as seeds or eggs? Longevity seems more related to an organism's complexity, how much energy it spends on reproduction, and its overall activity level – factors likely shaped by natural selection itself.

Ancient Egyptian animals haven't changed? Some argue that since animals depicted or preserved from ancient Egypt (3,000-4,000 years ago) look like modern ones, no species anywhere have changed. But this argument proves too much. Domestic animals from ancient Egypt also look like modern breeds, yet we know they were produced by modifying wild ancestors. A stronger argument against change would be animals that haven't changed since the last Ice Age – they experienced huge climate shifts and migrations, whereas Egypt's conditions have been quite stable. Lack of change doesn't hurt the theory of natural selection (which requires beneficial variations and favorable circumstances); it only challenges ideas about life having an built-in, automatic drive to change or progress.

More Objections and Responses

Objection: How can a variety live alongside its parent species? (Bronn)

Answer: They could coexist if they adapt to slightly different ways of life or different parts of the environment (e.g., high vs. low ground, wet vs. dry areas). For animals that move around and interbreed easily, varieties are usually found in distinct geographical regions.

Objection: Species differ in many parts, not just one. How can selection change many things simultaneously? (Bronn)

Answer: They don't have to change simultaneously. Important changes can happen step-by-step: first, a useful small change in one part is selected, then later, a useful change in another part. Since all these changes are passed down together, they appear to us as if they happened at the same time. Domestic animals show this: racehorses and draft horses differ greatly overall, but the transformation happened gradually, improving one part then another. Even when humans select for just one feature (like a flower), other parts of the plant usually change slightly too, due to linked growth (correlation) or other variations.

Objection: Many traits seem useless. How could selection create them? (Bronn, Broca, Nägeli)

This is a serious challenge. Critics point to things like different ear/tail lengths in rodents, complex tooth enamel patterns, and many plant features (cell arrangements, leaf patterns, flower parts, ovule position, some seed shapes) that seem unimportant for survival. Nägeli believed plant families differ mostly in these "useless" traits and suggested an internal drive towards perfection.

Response 1: We Don't Always Know What's Useful. We should be very cautious about deciding a structure is useless. The function of many important organs was unknown until recently. (e.g., mouse ears are highly sensitive touch organs; mouse tails can be used for grasping).

Response 2: Linked Changes ("Laws of Growth"). Modifying one part often causes changes in others due to complex developmental links (nutrient flow, pressure, timing, unknown correlations).

Response 3: Direct Effects & Spontaneous Variation. Environmental conditions can have direct effects. Also, "spontaneous" variations occur (like a new rose type on an old bush) seemingly unrelated to conditions. Every variation has a cause; if consistent, it could modify the whole species.

Self-Correction: I might have previously underestimated changes due to spontaneous variability. But we can't attribute the countless perfect adaptations we see solely to this cause. That's like saying a racehorse's sleek form arose by chance without selection.

Plant Examples: Many orchid structures once seemed purely decorative but are now known to be vital for insect pollination, likely shaped by selection. Different flower forms on the same plant (dimorphic/trimorphic) are crucial for proper fertilization. Ovule position might affect which ones get fertilized. Closed, self-pollinating flowers often have reduced parts (petals, stamens); selection likely favored saving resources, but the specific structural changes probably result from growth laws during this reduction.

Growth Law Examples (Position Effects): Parts differ based on position: leaf angles on branches; central vs. side flowers having different part numbers; outer vs. central flowers differing in petal size or seed features (sometimes used in classification!). These result from growth/interaction, not selection (except maybe showy outer flowers attracting insects).

Other Unexplained Variations: Sometimes important structures vary randomly between flowers or plants under identical conditions, with no clear use. These variations (e.g., flower part numbers, internal structures) likely aren't signs of "progression" but rather show these traits are unimportant to the plant's survival, allowing them to fluctuate.

What Happens to Unimportant Traits?

Natural selection doesn't accumulate variations in useless traits. (Useless parts tend to become variable).

However, unimportant modifications arising from growth laws can be inherited stably for long periods (e.g., hair on mammals, feathers on birds).

Result: Traits common to large groups are ranked as important by scientists, even if they have little importance for the species' actual survival. Morphological details might become fixed without selection being involved. The Paradox: Characters crucial for classification might be trivial for the organism's life. (This makes sense under common descent).

Progressive Development: There's no proof of an internal drive towards perfection. However, natural selection itself tends to lead to progress (usually), because specializing parts for different functions (higher organization) is generally advantageous and thus favored.

Responding to Mr. Mivart's Objections

Mr. St. George Mivart recently collected objections against natural selection. His arguments are skillful but often ignore counter-points like the effects of use/disuse or variation independent of selection (both of which I emphasize). After reviewing his points, I feel more convinced of my theory's general truth.

Mivart's Key Objection: How Do Useful Structures Begin? He argues selection can't explain the "incipient stages" before a structure is fully useful. This involves gradations and changes in function. Let's revisit his examples:

Giraffe's Neck: Adapted for Browse high. Selection favors individuals reaching slightly higher during food shortages. Small variations in body part lengths exist. Selection preserves longer-necked/legged individuals; they breed; inherited effects of use strengthen parts. Gradual process seems very likely.

Objection 1 (Food): Bigger body needs more food? Counter: Giraffes and other large antelope thrive in Africa. Reaching untouched food is a big advantage. Bulk protects from predators; neck is watchtower and weapon.

Objection 2 (Why only giraffe?): Why haven't others evolved long necks/trunks? Counter: Competition is giraffe-vs-giraffe for high branches. Elsewhere? We don't know all factors preventing it (conditions maybe didn't favor large size, needed parts didn't vary right, other pressures dominant, not enough time). Expecting specific answers is unreasonable.

Other Non-Developments: Why no flight for ostrich? (Needs immense energy). Why no land mammals from seals/bats on islands? (No prey for seal-carnivore; bats face existing competition, ill-suited for ground life needed first). Transitions need specific conditions and gradual benefits.

Mimicry: Mivart questions how tiny variations build resemblance. Counter: Starts with accidental resemblance -> selection preserves any variation making it slightly better -> perfection refined over time because predators have sharp eyes. Variations most likely in traits already differing between species.

Whale Baleen: How did filter-feeding plates begin? Counter: Imagine ancestor with mouth like duck's beak (lamellae for sifting). Gradations exist in ducks (goose -> duck -> shoveller), showing plausible steps from grasping/tearing to filtering. Selection could favor similar steps in early whales.

Flatfish Eyes: How could eye migration start if initially harmful? Counter (Malm): Young fish fall on side, habitually strain lower eye up -> flexible skull yields -> eye moves. Use + inheritance + selection perfect it.

Prehensile Tails: How could first slight grasping help? Counter: Habit likely enough. Young monkeys use tails; non-prehensile mice use tails for climbing aid. Habit could lead to structural adaptation.

Mammary Glands: Young saved by sucking gland? Counter: Mammals likely from marsupials -> glands maybe first nourished young in pouch (like Hippocampus fish?) -> selection improved best secreting glands -> concentrated into breast. Learning to suck no harder than chick hatching. Larynx change maybe needed for voice/swallowing solids.

Echinoderm Pedicellariæ (Pincers): Utility of beginnings? Counter: Developed from modified defensive spines; clear gradations exist from simple spine to stalked pincer, each step useful.

Polyzoa Organs (Avicularia/Vibracula): Different organs from same source (zooid/cell). Gradations likely existed/were useful.

Orchids/Climbing Plants: Clear, useful gradations exist for pollinia and climbing mechanisms, originating from simpler structures/abilities enhanced by selection.

Summary of Transitions: Gradual steps are plausible for all these cases, involving selection, use/disuse, habit, and changing functions.

Why Don't All Species Get Helpful Structures? We can't know specifics. Maybe needed variations didn't occur, other pressures interfered, conditions weren't right long enough. Selection is complex. Mivart's points seem less weighty than evidence favoring selection.

Alternative Ideas: Internal Force & Sudden Changes

Mivart suggests species change via an unknown "internal force/tendency." I argue normal variability + selection is sufficient.

Mivart also suggests species appear "with suddenness." Examples: Hipparion to horse; bird wings appearing abruptly.

Why I Disbelieve Abrupt Changes:

Domestic abrupt variations are rare, often single individuals (lost by chance/crossing unless human protects). Nature would need multiple individuals changing simultaneously (unlikely). Gradualism avoids this by preserving many slightly varying individuals.

Abundant evidence points to gradual change (closely related species/genera, representative forms, fossils, fine gradations in organs).

Large groups of facts only make sense with small steps (patterns of variation in large vs. small genera, specific vs. generic characters, etc.).

Embryology is strong evidence against leaps. Wings and legs start indistinguishable, differentiate gradually. Embryos resemble ancestral forms. Incredible that huge, sudden changes would leave no trace on development.

Sudden change via internal force requires simultaneous variation, explains nothing about complex co-adaptations, ignores embryology -> seems like invoking miracles, not science.

Final Points of the Chapter

"Nature Makes No Leaps": The old saying isn't strictly true for living forms today, but if we include all past life, the theory of natural selection implies it must be true overall. Selection works by small, slow steps.

Two Great Laws Explained:

Unity of Type: The fundamental similarity in structure among organisms of the same class (like the bones in a human arm, horse leg, bat wing, seal flipper), regardless of lifestyle. Explained by inheritance from a common ancestor.

Conditions of Existence: The idea that organisms are adapted to their environment (stressed by Cuvier). Fully embraced by natural selection, which adapts organisms to their conditions over time, aided by use/disuse, environmental effects, and growth laws.

Conclusion: The "Conditions of Existence" is the broader law, as it includes "Unity of Type" through the inheritance of past adaptations.

Crossing Species: Hybrids, Sterility, and the Line Between Species and Varieties

This chapter tackles a major challenge: why can different varieties of the same species usually breed together easily, while different species often cannot, or produce sterile offspring like mules (hybrids)?

The Common Belief (and Why It Might Be Wrong)

Many scientists believe that species were intentionally created ("specially endowed") with sterility between them. The idea is this prevents different species from mixing together and becoming confused.

This seems logical at first. If species living together could freely interbreed, how would they stay distinct?

However, this topic is crucial because I believe this sterility wasn't intentionally created. It's an accidental side effect of other differences that evolved between species' reproductive systems.

Importantly, this sterility cannot have been built up gradually by natural selection favoring individuals that were slightly less fertile with other groups.

Two Kinds of Breeding Problems

It's important to distinguish between two related but different issues people often mix up:

Difficulty Making the First Cross: Sometimes, two pure species simply won't produce offspring together, or only do so with great difficulty, even though their own reproductive organs seem perfect. The problem lies in the sexual elements (sperm/pollen and egg) failing to combine or develop properly.

Sterility of the Hybrid Offspring: Other times, two species can produce offspring (hybrids), but those hybrids have reproductive organs that don't function correctly. They look fine under a microscope, but they can't produce working sperm/pollen or eggs.

This distinction matters when we try to figure out the cause of sterility. People might have overlooked it because they thought sterility was a deliberate act of creation, not something explainable by science.

The Fertility of Varieties

The flip side of this issue is just as important for my theory: different varieties of the same species are generally fertile when crossed, and their mixed offspring (mongrels) are also fertile. This seems to draw a clear line between varieties and species. Or does it?

How Sterile Are Species Crosses, Really?

Two legendary researchers, Kölreuter and Gärtner, spent their lives studying plant hybridization. Their work shows that some degree of sterility is extremely common when crossing different species.

Kölreuter thought it was a universal rule (but if two forms bred freely, he just called them varieties, solving the problem by definition!).

Gärtner also thought it was universal. He disputed Kölreuter's supposedly fertile cases, often by carefully counting seeds to show even a slight reduction in fertility compared to the parent species.

Problems with Measuring Sterility: Gärtner's methods might have sometimes caused sterility. To make a cross, plants must be altered (castrated) and protected from unwanted pollen, often by being potted indoors. Gärtner himself showed these processes could reduce a plant's fertility even when pollinated with its own pollen. Also, Gärtner found that some forms widely considered varieties (like the blue and red pimpernel) were completely sterile together, suggesting his standard might have overestimated sterility in other cases.

Sterility Isn't Black and White

The reality is:

Sterility between species varies hugely, from absolute inability to cross, to slightly reduced seed numbers, to full fertility. It's a gradient, not an all-or-nothing thing.

The fertility of pure species itself is easily affected by environmental conditions.

Result: It's practically impossible to draw a sharp line where perfect fertility ends and sterility begins.

Proof: Kölreuter and Gärtner reached opposite conclusions about the fertility of the very same plant crosses! Comparing different botanists' opinions on whether forms are species or varieties with hybridizers' fertility results shows massive disagreement and uncertainty.

Conclusion: Fertility or sterility is not a reliable, definite test to distinguish species from varieties. The evidence from breeding results is just as graduated and uncertain as evidence from physical structure.

Sterility of Hybrids Over Generations

Gärtner found that even when hybrids could reproduce, their fertility didn't increase over generations (he followed some for 6-10 generations) but usually decreased sharply.

Why? Part of the reason might be that if both parents have slightly affected reproductive systems, this effect is magnified in the offspring.

Author's View: Close Interbreeding is the Main Culprit. My own experiments and many facts show that crossing with different individuals boosts vigor and fertility, while close interbreeding (siblings mating) reduces them. Hybrid experiments usually involve small numbers, carefully protected from outside pollen. This means hybrids often self-pollinate or cross with siblings, leading to decreased fertility due to inbreeding, on top of their inherent hybrid issues.

Gärtner's Evidence Supports This? Gärtner repeatedly noted that artificially fertilizing less fertile hybrids (using pollen from the same hybrid type) sometimes increased their fertility over generations. Why? Because artificial pollination often involves taking pollen by chance from another flower (maybe on the same plant, maybe another hybrid plant). This avoids self-pollination and ensures a cross, preventing the worst effects of close interbreeding.

A Different View: Hybrids Can Be Fully Fertile

Another expert hybridizer, W. Herbert, strongly concluded that some hybrids are perfectly fertile, just like their parent species.

He worked with some of the same species as Gärtner but got different results, perhaps due to better horticultural skills and facilities (hot-houses).

Example: A Crinum lily cross produced a pod where every single seed grew – better fertility than the parent species showed naturally!

Strange Cases: Easier to Cross Than Self-Fertilize!

Some plants (Lobelia, Verbascum, Passiflora, Hippeastrum, Corydalis, some orchids) show a bizarre pattern:

Individuals can be easily fertilized by pollen from a different species.

But they cannot be fertilized by their own pollen (even though the pollen is perfectly good and can fertilize other plants).

In some species, all individuals are like this!

Example: Herbert's Hippeastrum bulb: 3 flowers self-pollinated died; 1 flower pollinated by a complex hybrid produced vigorous seeds.

Conclusion: This shows that fertility can depend on very slight and mysterious causes within the reproductive system.

Evidence from Gardeners

Practical gardeners, though not doing precise science, provide useful insights.

Complex hybrids of popular garden plants (Pelargonium, Fuchsia, Calceolaria, Petunia, Rhododendron) are often highly fertile. Herbert's Calceolaria hybrid reproduced "as perfectly as if it had been a natural species." Mr. C. Noble's hybrid rhododendron "seeds as freely as it is possible to imagine."

If hybrid fertility always decreased, nurserymen would know. They grow large beds of the same hybrid, allowing insects to cross-pollinate them freely, preventing close interbreeding.

Hybrid Animals

Fewer careful experiments have been done with animals, often because they don't breed well in captivity.

If animal classifications are reliable, it seems animals from more widely distinct groups can sometimes be crossed more easily than plants, but the resulting hybrids might be more sterile.

Example: Canaries crossed with 9 different finch species – but since none breed well in captivity, we can't expect fertile crosses or hybrids.

Inbreeding Problem: Hardly any cases exist where hybrid animals were bred for generations without close interbreeding (crossing brothers/sisters), making it hard to judge their true potential fertility.

Fertile Animal Hybrids? Few proven cases, but possibly: hybrids of Cervulus deer; hybrids of Phasianus pheasants. Moths (Bombyx) reportedly fertile for 8 generations. Hare x rabbit offspring said to be fertile with parent species. Geese: Common goose x Chinese goose (often put in different genera) have bred with parents, and once bred with each other (Mr. Eyton raised 8 hybrid offspring from two hybrid parents). In India, flocks of these crossed geese are kept for profit where pure parents aren't present, proving they must be highly or perfectly fertile there (Mr. Blyth, Capt. Hutton).

Domestication Seems to Remove Sterility

Domestic animal breeds are fully fertile when crossed, yet many likely descended from different wild species (e.g., dogs, cattle, pigs).

This implies either:

The original wild species produced fertile hybrids (less likely).

Hybrids became fertile under domestication (Pallas's idea, seems probable).

Conclusion: We must either abandon the idea that all species crosses are sterile, or accept that sterility isn't permanent and can be removed by domestication.

Overall Conclusion on Sterility: Some degree of sterility in first crosses and hybrids is very common, but not absolute or universal.

Laws Governing Sterility: Any Sign of Special Design?

Let's examine the rules of sterility (mainly from Gärtner on plants, but largely applicable to animals too) to see if they suggest a special "design" to keep species separate.

Graduated Fertility: Fertility ranges smoothly from zero to perfect (or even excess). Hybrids range from totally sterile to fully fertile.

First Cross vs. Hybrid Sterility: The difficulty of making the first cross doesn't always match the sterility of the resulting hybrid. Easy cross -> sterile hybrid; difficult cross -> fertile hybrid. Both happen.

Conditions Matter: Fertility (first cross & hybrid) is more sensitive to unfavorable conditions than pure species fertility.

Innate Variability: Fertility degree varies between individuals used in the same cross, and between hybrid siblings raised from the same seed pod.

Systematic Affinity (Relatedness): Fertility is largely related to how closely species are related. No hybrids between distinct families. Close species usually cross easily. BUT relationship isn't strict: Many close species won't cross; many distinct ones cross easily. Some genera cross readily (Dianthus), others barely (Silene). Even within a genus, some species cross easily, others resist (Nicotiana).

What Prevents Crossing? No known type or amount of difference guarantees cross failure. Widely different plants often cross easily.

Reciprocal Crosses: Results often differ hugely depending on which species is male/female (Mirabilis example). This difference is common, even between varieties. Crucially: This proves crossing ability depends specifically on the reproductive systems, not just overall similarity. Hybrids from reciprocal crosses also often differ in fertility.

Do These Rules Suggest Sterility Was Intentionally Created?

I think not.

Why would the degree of sterility vary so much if the goal was simply to keep all species separate?

Why would sterility be innately variable?

Why the mismatch between crossing ease and hybrid fertility?

Why the difference in reciprocal crosses?

Why allow hybrids to be produced at all if they are meant to be dead ends?
It seems like a strange, inconsistent way to prevent species from blending.

Alternative View: Sterility is an Accidental Side Effect

These complex rules make more sense if sterility (both first cross and hybrid) is simply an incidental outcome of unknown differences in the reproductive systems that arise as species diverge. These differences are peculiar, sometimes allowing fertilization one way but not the other (reciprocal crosses).

Analogy: Grafting Plants: A plant's ability to be grafted onto another isn't vital in nature, so nobody thinks it's a special endowment. It's incidental to differences in their growth systems (growth rate, wood hardness, sap). We often don't know why grafts fail. Grafting success loosely follows relatedness, but with many exceptions (like crossing). Reciprocal grafts also show differences. Grafting can even affect fertility.

Parallelism: Though grafting (tissue joining) and crossing (sexual element union) are different, their complex rules show parallels. Conclusion: Just as grafting success depends incidentally on vegetative systems, crossing success depends incidentally on reproductive systems. Both loosely follow overall similarity (systematic affinity). The difficulty isn't a special endowment, though crucial for species stability (unlike grafting).

Could Natural Selection Have Created Sterility Directly?

At first, it seemed possible that selection might favor slightly reduced fertility between diverging varieties to prevent blending (like breeders separate stocks).

Problem 1 (Distant Species): Species living far apart are often sterile when crossed. Selection couldn't have done this, as there was no advantage.

Problem 2 (Reciprocal Differences): One-way sterility seems disadvantageous, unlikely to be selected.

Problem 3 (Gradual Increase): How could selection increase sterility step-by-step? If a cross produces few sterile offspring, what advantage does an individual gain by being even less fertile with that other variety? There's no benefit to itself or its group (unlike sterile insects benefiting their colony). Yet, selection would need to constantly favor increasing infertility to reach the absolute sterility common between distinct species.

Conclusive Evidence (Plants): Plant genera show series from reduced seed set -> zero seeds but ovary affected. Impossible to select more sterile individuals if already producing zero seeds. Therefore, this high degree of sterility cannot be due to selection. Since sterility laws seem similar across plants/animals, the cause is likely the same, and not selection.

What Causes Sterility? (Looking Closer)

First Crosses: Difficulty seems due to several factors:

Physical impossibility (pollen tube too short).

Pollen fails to penetrate stigma.

Fertilization happens, but embryo fails to develop.

Embryo dies early: This seems very common. Pheasant/fowl crosses (Hewitt): many eggs fertilized, but embryos died early, or chicks too weak to hatch/live. Hybrid willows weak/dwarfed (Wichura). Why? Hybrid is only half like mother; conditions in womb/egg might be unsuitable. More likely: imperfect fertilization creates flawed embryo.

Hybrids: Sexual elements (pollen/sperm, eggs) develop imperfectly.

Parallel to Changed Conditions: Removing organisms from natural conditions often causes sterility. Similarities:

Sterility often unrelated to general health.

Often linked to large size or vigorous growth.

Occurs in various degrees; male element often affected most.

Runs loosely with relatedness (whole groups affected similarly).

Exceptions exist (some species resist change; some crosses yield fertile hybrids).

Unpredictable who will breed in captivity or which cross works.

Link to Variability: Organisms under unnatural conditions tend to vary. Hybrids' reproductive systems are disturbed -> their offspring are highly variable.

Conclusion: Unnatural conditions OR unnatural crossing (blending two different constitutions/systems) disturbs the reproductive system similarly. Hybrids inherit this compounded, disturbed organization -> sterility continues (or worsens with inbreeding).

Mysteries Remain: Can't explain unequal reciprocal hybrid fertility, or why sterile hybrids sometimes closely resemble one parent. Don't know ultimate cause of sterility from unnatural conditions. Point: Sterility is a common result in both allied cases (disturbed conditions; disturbed blended organization).

Another Parallelism: Benefit of Slight Change vs. Harm of Large Change

Benefit: Slight changes in conditions benefit organisms. Slight crosses (between slightly different individuals/varieties) boost offspring vigor/fertility.

Harm: Large changes in conditions often cause sterility. Large crosses (between distinct species) often cause sterility.

Connection? This double parallelism seems linked by some fundamental principle of life (maybe related to Herbert Spencer's idea of life responding to disturbed equilibrium). Explaining why elephants are sterile in captivity might unlock key to hybrid sterility, and why domestic races (from distinct species) are fertile together.

Insights from Dimorphic and Trimorphic Plants

These plants (with 2 or 3 forms differing in sex organ lengths) shed light:

Need "legitimate" cross (pollen from stamen matching pistil height on different form) for full fertility. "Illegitimate" crosses less fertile/sterile.

Parallels: Illegitimate unions behave exactly like species crosses regarding sterility degrees, conditions effect, pollen prepotency, reciprocal differences.

Illegitimate Offspring = Hybrids: Seedlings from illegitimate unions are infertile like hybrids when crossed among themselves. Crossing illegitimate x legitimate reduces sterility (like hybrid x parent).

Conclusions from Dimorphism:

Fertility isn't a reliable test for species distinction.

Unknown bond links fertility of first union and offspring (in both illegitimate unions and species crosses).

Crucially: Forms of the same species, identical except reproductive organs, can be sterile when united certain ways. Suggests sterility depends only on sexual element nature, not general structure/constitution. Reciprocal crosses also point to this. Gärtner reached same conclusion.

Why Are Varieties Usually Fertile When Crossed?

This seems like the key distinction between varieties and species.

General Rule: Admit varieties usually fully fertile (with exceptions).

The Circle Argument: But if naturally occurring varieties are found sterile, they are immediately reclassified as species (like the pimpernel). This makes fertility of natural varieties guaranteed by definition.

Domestic Varieties: Perfect fertility of distinct pigeon or cabbage varieties is remarkable compared to sterile but similar wild species. Why?

External difference isn't a reliable guide to sterility anyway.

Sterility depends on sexual constitution.

Domestication conditions seem to eliminate sterility tendencies (Pallas doctrine), not create them. Descendants of possibly sterile wild species become fertile together. (Cultivation sometimes makes plants self-sterile but cross-fertile with others).

If domestication eliminates sterility, unlikely it also induces it.

Conclusion: Understandable why domestic varieties aren't mutually sterile.

The Real Difficulty: Not why domestic varieties are fertile, but why natural varieties become sterile once they differ enough to be called species. Cause unknown. Maybe long periods under uniform conditions make wild species' reproductive systems more sensitive to the "unnatural" disturbance of crossing? Domesticated organisms are less sensitive to change (that's why they could be domesticated) and might produce varieties less affected by crossing.

Exceptions: Sterile Variety Crosses Do Exist

It's not true varieties are always fertile. Evidence for some sterility is strong (from hostile witnesses who normally use fertility as species test):

Maize: Dwarf yellow x tall red varieties barely crossed naturally; artificial cross yielded only 5 grains (hybrids fertile). Still considered varieties.

Gourds: Crossing harder between more different varieties (Girou). (Considered varieties).

Verbascum: Yellow x white varieties yield less seed than same-color crosses (Gärtner). Crosses between varieties of different species yield more seed if colors matched! (Scott partly confirms same-species result). Varieties differ only in flower color.

Tobacco: One specific variety crossed with N. glutinosa yielded less sterile hybrids than 4 other varieties did (Kölreuter). Its reproductive system was modified somehow.

Final Thoughts on Fertility Distinction

Varieties are NOT invariably fertile when crossed.

Given difficulty finding sterile natural varieties (reclassified), human focus on external traits, less uniform conditions for domesticates -> fertility isn't a fundamental species/variety distinction.

Sterility of crossed species likely incidental on unknown sexual element changes.

Comparing Hybrids and Mongrels (Other Aspects)

Besides fertility, how do offspring of species crosses (hybrids) and variety crosses (mongrels) compare? Gärtner (wanting a clear line) found few/unimportant differences; author sees close agreement.

Variability: Mongrels maybe more variable F1 (parents = varieties, recent variability continues). Hybrids F1 less variable (parents = stable species). Hybrids variable F2+ (reproductive systems disturbed). Overall difference graduates away.

Reversion: Mongrels revert more? Difference only in degree. Hybrids from cultivated plants revert more (explains observer disagreements?).

Crossing with Third Species: Gärtner's claim hybrids differ widely, mongrels little seems based on weak evidence, possibly wrong.

Similarities: Resemblance to parents follows same laws. Prepotency occurs in both. Reciprocal cross offspring similar. Both reducible by backcrossing. (Animal cases complex: sex differences, prepotency varies by sex - ass dominant over horse, male ass > female ass).

Resembling One Parent: Happens sometimes in hybrids (less often than mongrels). Seems confined to sudden/monstrous traits. More likely in mongrels from suddenly-produced varieties. Overall Agreement (Lucas): Laws of resemblance same whether parents differ little or much.

Conclusion: Hybrids and mongrels remarkably similar in all aspects except (usually) fertility. Astonishing if species created, varieties secondary. Harmonizes perfectly if no essential species/variety distinction.

Chapter Summary: Hybridism

First crosses (species) & hybrids very generally, but not universally, sterile.

Sterility graduated, variable, affected by conditions. Not strictly linked to relatedness. Often differs in reciprocal crosses.

Sterility not special endowment to prevent blending, but incidental result of unknown reproductive system differences (like grafting capacity).

Sterility not acquired by natural selection (cannot select for increased infertility). Cause likely related to disturbance (conditions or blending constitutions).

Parallelism: Slight change/cross beneficial; large change/cross causes sterility.

Dimorphic/trimorphic plants confirm sterility depends on sexual elements; fertility no safe species test.

Varieties generally fertile crossed, but exceptions exist. Understandable via domestication effects (eliminating sterility). Fertility not fundamental distinction.

Hybrids & mongrels closely similar in all respects (variability, reversion, inheritance) except (usually) fertility.

Overall: Facts align with belief species originated as varieties, not special creations.

Dealing with Gaps: Evolution and the Fossil Record

Many questions probably came up as you read the previous chapters. Some are so challenging they still make me question things. But I believe most difficulties are only apparent, and the real ones don't destroy the theory.

This chapter focuses on objections related to geology and the fossil record.

The Main Geological Challenge: Where Are the Innumerable Links?

One obvious difficulty was discussed before: Why do species look distinct today? Why don't we see countless transitional forms blending them all together?

I explained that competition often creates sharp boundaries between species.

Intermediate varieties likely exist in smaller numbers and get outcompeted.

Most importantly, natural selection itself tends to exterminate the parent forms and the intermediate links as new, improved forms arise.

But if this process has happened on a massive scale, there must have been an enormous number of intermediate varieties in the past.

The Big Question: Why isn't every geological formation and every rock layer full of fossils of these intermediate links? Geology certainly doesn't show us such a finely graded chain of life.

This is perhaps the most obvious and serious objection to the theory.

The Answer: I believe the explanation lies in the extreme imperfection of the geological record.

What Kind of Fossil Links Should We Expect?

It's important to understand what kind of "intermediate" forms my theory predicts.

We shouldn't picture forms directly between two living species, like halfway between a horse and a tapir.

Instead, we should look for forms intermediate between each living species and their common, unknown ancestor.

This ancestor would resemble both descendants in general ways but likely differ significantly in others.

Pigeon Example: Fantail and Pouter pigeons both came from the Rock Pigeon. If we had all the intermediate fossils, we'd see links from Fantail back to the Rock Pigeon, and from Pouter back to the Rock Pigeon. We would not find fossils directly intermediate between a Fantail and a Pouter (e.g., slightly bigger crop AND slightly bigger tail). They diverged from a common point.

Result: Even if we found the fossil of the common ancestor, we might not recognize it as such without the whole chain of intermediate links connecting it to its descendants.

(It's technically possible one living species descended directly from another living one, meaning direct links existed. But this would mean one form stayed unchanged for ages while the other changed vastly. Competition usually makes the new, improved forms replace the old ones, so this scenario is likely very rare).

The Huge Number of Missing Links

My theory requires that all living and extinct species are connected by steps no bigger than the differences between varieties we see today. This means the number of intermediate, transitional links that must have existed is "inconceivably great." If the theory is true, they definitely lived on Earth.

Objection: Has There Been Enough Time?

Another objection: Could so much slow change have happened in the time Earth has existed?

It's hard to grasp geological time. Reading Sir Charles Lyell's Principles of Geology helps. We need to appreciate the evidence:

Watch erosion happening now (streams carrying mud, waves wearing cliffs).

Consider the immense thickness of sedimentary rock layers piled on top of each other (in Britain, nearly 14 miles thick!).

Understand that these rocks are the result of unimaginable amounts of erosion elsewhere.

Realize there are likely huge time gaps between these rock formations.

Coastal Erosion is Slow: Cliffs erode only where tides reach, and mostly when waves carry sand/pebbles. Boulders at cliff bases often covered in marine life, showing they aren't moved or worn down much. Erosion happens only in small sections of a coastline at any given time.

Land Surface Erosion (Subaerial Denudation) is More Important: Chemical action (air, rainwater), frost, wind (especially in dry areas), and rivers constantly wear down the land surface. Muddy streams on a rainy day show this process. Large escarpments (like in the Weald, UK) weren't ancient sea coasts, but rather harder rock layers left standing as the surrounding softer rock eroded away over vast time (Ramsay, Whitaker). This slow process having huge results powerfully impresses the mind with the immensity of past time.

Evidence from Uplift and Faults: Volcanic islands worn into high cliffs show how far land once extended. Fault lines (huge cracks where land moved up/down thousands of feet) are often completely invisible on the surface today because erosion has planed everything smooth. This smoothing required enormous amounts of time.

Putting Geological Time in Perspective (Time in Years)

Trying to imagine millions of years is difficult. Mr. Croll calculated that rivers might erode about 1000 feet of rock from the average land surface in about 6 million years. Even if halved or quartered, it's still vast erosion over that time.

Croll's Illustration: If a paper strip 83 feet long represents a million years, then 100 years is just one-tenth of an inch.

What 100 Years Means for Evolution: In a single lifetime (say 50 years), breeders have significantly changed domestic animals. 100 years is two breeders' lifetimes. Even "unconscious selection" (just keeping the best without trying to change the breed) noticeably alters breeds in 2-3 centuries.

Species Change Slower: Natural species probably change much more slowly than domestic ones under selection. Only a few species in an area likely change at the same time. Slowness results because existing species are already well-adapted; new ecological "jobs" or niches open up only after long intervals (due to climate change, immigrations, or other species slowly changing). The right variations don't always appear immediately when needed. We have no way to measure the rate of species change in years.

Why the Fossil Record is So Poor

Now, let's look at why we don't find those countless intermediate fossils.

Poor Collections: Our fossil collections are paltry! Everyone agrees. Many species named from single, broken specimens (E. Forbes). Only small parts of the world explored geologically, none perfectly.

Preservation Issues:

Soft-bodied organisms rarely preserve.

Shells/bones decay on the seabed if not buried quickly by sediment.

Sediment doesn't accumulate fast enough everywhere to preserve remains (clear blue ocean water indicates purity, little sediment).

Seabed often remains unchanged for ages (shown by later formations resting directly on much older ones with no erosion between).

Remains buried in sand/gravel often dissolve later when uplifted land lets rainwater seep through.

Beach organisms rarely preserved (e.g., Chthamalus barnacles abundant today, existed in Chalk period, yet almost no fossils found).

Many huge rock formations lack fossils entirely for unknown reasons (e.g., Flysch formation).

Terrestrial Record Worse: Fossils of land life from older periods (Secondary, Paleozoic) are extremely rare. Land shells, mammals mostly found in caves or lake deposits, which aren't known from those older eras.

Gaps Between Formations (Intermittence): This is the most important reason for imperfection. Formations are almost always separated by huge intervals of time.

Written tables make formations look continuous, but geology in places like Russia and North America shows vast gaps. A geologist studying only one region wouldn't know that huge deposits with different life forms were accumulating elsewhere during their region's "blank" periods.

Different rock types in successive formations imply big geographical changes and vast time gaps between them.

Why Formations are Intermittent (Process of Deposition):

Looking at recently uplifted coasts (like South America), extensive recent deposits are often missing. Why? Because as land slowly rises, deposits near the shore are constantly worn away by waves.

To survive erosion, sediment needs to accumulate in thick, extensive masses. This happens mainly in two ways:

In deep seas: But deep-sea life is less abundant and varied, giving an imperfect record of nearby life.

On shallow bottoms during slow subsidence (sinking): If sediment supply keeps pace with sinking, the sea stays shallow and full of diverse life. This creates thick, fossil-rich formations that can survive later erosion.

Conclusion: Most ancient, fossil-rich formations likely formed during periods of subsidence. (Many geologists agree).

During periods when the seabed is stationary, thick deposits can't form in shallow, life-rich areas.

During periods of elevation (uplift), any deposits formed are usually quickly destroyed by erosion as they are brought near the coast.

(Shallow seas might form thinner deposits during elevation, but these are less likely to survive long-term).

Complete Denudation: It seems likely that sometimes entire formations have been completely worn away, leaving no trace. This is suggested by vast areas where ancient metamorphic and granitic rocks (which must have formed deep underground) are now exposed at the surface. The original overlying rocks have been completely stripped away.

How Geological Processes Affect the Record of Evolution

Elevation Periods: Land area increases, new habitats form. Favorable for new varieties/species to arise. BUT geological record is usually blank during these periods.

Subsidence Periods: Land area decreases, populations shrink, much extinction occurs, few new forms arise. BUT this is when the richest fossil deposits are formed.

Summary of Record Imperfection: The geological record, viewed as a whole, is extremely incomplete due to all these factors.

Why Links Are Missing Within a Single Formation

Even within one formation (representing a long time), why don't we usually find finely graded links between species found at the bottom and top?

Time Scale: A formation's timespan, though long, might be short compared to the time needed to transform one species into another. (Hard to compare these durations accurately).

Migration: Species appearing or disappearing within a formation likely migrated in or out, rather than evolving or going extinct right there. Glacial period shows massive migrations occurred within one geological period. Deposits during such times likely show species coming and going due to migration and geography changes, not complete evolution in one spot.

Continuous Deposition Rare: To capture a perfect gradation, deposition must be continuous for a very long time, and the species must live there the whole time. Thick, continuous fossiliferous deposition requires a rare balance of subsidence and sediment supply. Even then, deposition likely intermittent (different rock beds, fossil trees show pauses/level changes). Species probably disappeared and reappeared locally multiple times within one formation's timespan. This breaks the record of transitions.

Species Definition Issues: We rank fossils as distinct species based on slight differences, especially if from different levels. We wouldn't recognize parent-child relationships without numerous linking varieties, which are rarely preserved.

Local Origin of Varieties: Varieties likely start locally and spread only after becoming well-established. We're unlikely to find the very earliest transitional stages in any one spot's fossil record. Widely-ranging marine species likely produced local varieties far away, which then migrated back changed -> appear as new species.

Short Modification Periods: Species probably spend most of their existence unchanged, with relatively short periods of modification (Falconer). Reduces chance of finding fossils during the change.

Conclusion on Missing Fossil Links: Given all these reasons, finding numerous, fine, intermediate fossil links is highly improbable, even between species in the same formation. We shouldn't expect it. The assertion that geology yields no linking forms is wrong (more next chapter), but the absence of infinite fine gradations is understandable due to the record's imperfection.

Objection: Sudden Appearance of Whole Groups of Species

Paleontologists like Agassiz, Pictet, Sedgwick argued the sudden appearance of whole groups (genera, families) in certain formations is fatal to the theory of gradual evolution.

Response: This assumes the geological record is far more complete than it is. We falsely assume groups didn't exist before we first find their fossils. Negative evidence is worthless. The world is vast; groups could have existed elsewhere for ages, slowly evolving and multiplying, before invading the European/US areas where we have studied formations. Huge time gaps between formations allow ample time for diversification from a single ancestor, making groups appear suddenly created in the next preserved layer.

Adaptation Time: It might take a very long time for a group to adapt to a completely new way of life (like flight). Transitional forms might be confined to one region for ages. Once perfected, the group might diversify and spread rapidly. (Penguins show an intermediate wing state; maybe their descendants could evolve flapping, then full flight?).

Examples of Corrected "Sudden" Appearances: Paleontology constantly revises first appearances:

Mammals: Once thought Tertiary only -> now mid-Secondary, even earlier.

Monkeys: Once thought absent Tertiary -> now Miocene.

Bird-like reptiles: Known only from footprints in New Red Sandstone.

Birds: Once thought Eocene -> now Upper Greensand, plus Archeopteryx in older Jurassic rocks.

Author's Barnacle Example: Concluded sessile cirripedes appeared suddenly in Tertiary (abundant then, none found earlier). Troubled by this. Bosquet then found one (Chthamalus) in the Chalk (Secondary). Woodward found another (Pyrgoma). Group existed much earlier.

Teleost Fish Example: Often cited sudden appearance in Chalk. But earlier forms now classed as teleostean. Even if sudden in N. Hemisphere, doesn't mean worldwide simultaneous development (few fossils known elsewhere). Group could have developed in isolated sea (like Malay Archipelago if became enclosed basin), then spread when adapted to new climates.

Conclusion: Given ignorance of global geology and constant discoveries, it's rash to claim groups appeared suddenly worldwide based on limited evidence.

Most Serious Difficulty: Absence of Life Before the Cambrian

Species belonging to several major animal divisions appear suddenly in the lowest known fossil-rich rocks (Cambrian/Silurian).

Theory requires these groups descended from common ancestors living long before the Cambrian. (e.g., all trilobites from one ancestral crustacean). Ancient forms like Nautilus aren't intermediate ancestors.

Conclusion: If theory true, vast periods elapsed before Cambrian, teeming with life.

Objection (Physics): Has Earth been habitable long enough? Sir W. Thompson calculated Earth's crust solidified perhaps 98-200 million years ago (wide limits, data uncertain). Croll estimated ~60 million years since Cambrian. Seems short for amount of evolution. But Thompson suggests early Earth conditions changed faster -> faster evolution?

Why No Fossils Found Pre-Cambrian? No satisfactory answer. Views changed: Murchison (Silurian=first life) -> Barrande (lower stage) -> Hicks (rich Lower Cambrian beds). Phosphates/bitumen in oldest rocks suggest life? Eozoon in Laurentian (Canada) generally accepted (lived countless numbers, preyed on others). Logan: Laurentian strata thickness maybe > all later rocks combined; Cambrian fauna maybe "comparatively modern." Prediction of pre-Cambrian life proved true.

The Difficulty Remains: Why absence of vast fossil-rich strata below Cambrian? Unlikely completely destroyed/metamorphosed (next oldest formations usually not extremely altered).

Possible Hypothesis: Continents/oceans maybe swapped places long ago? Current continents near former landmasses (sediment source). Oceans existed where oceans are now? Maybe pre-Cambrian continents were where oceans are now. If Pacific floor became land -> would we find pre-Cambrian fossils? Maybe deep subsidence + water pressure caused extreme metamorphism, destroying record? Explains vast naked metamorphic areas? Case inexplicable currently; valid argument against theory.

Summary of Geological Objections

Absence of infinite fine transitional links.

Sudden appearance of groups in European formations.

Almost total lack of fossils beneath Cambrian.

All are serious difficulties. Explains why most top paleontologists/geologists long maintained species immutability. (Lyell now supports evolution; others reconsidering).

Those believing record perfect will reject theory.

Author's View (Lyell's Metaphor): Record = imperfectly kept history, changing dialect, last volume only (few countries), few chapters/pages/lines preserved. Words = life forms, falsely seem abrupt.

Conclusion: On this view of extreme imperfection, the geological difficulties are greatly diminished, or even disappear.

Evolution and the Story in the Rocks

Let's see how the facts from geology – the study of rocks and fossils – match up with the idea of evolution by natural selection, compared to the older view that species never change.

Patterns in the Fossil Record

New Species Appear Slowly and One by One: The fossil record, especially from more recent geological times (Tertiary period), shows that new species didn't all appear at once. They emerged gradually, one after another, over long periods. Each year, new fossil discoveries help fill the gaps, making the story of life appear more continuous. In the very latest rock layers, we find mostly species that are still alive today, with only one or two extinct species, and maybe one or two brand new ones appearing for the first time.

Different Groups Change at Different Rates: Not all types of life evolved at the same speed or to the same extent.

In older Tertiary rocks, you might find a few seashells identical to modern ones, alongside many extinct species.

An existing type of crocodile fossil was found alongside many extinct mammals in India (Falconer).

A type of shellfish called Lingula looks almost the same today as its ancestors from the very ancient Silurian period. But most other Silurian shellfish and all the crustaceans from that time have changed dramatically.

Land animals and plants seem to have changed faster than sea life.

Perhaps more complex organisms ("higher" on the scale) change faster than simpler ("lower") ones, though there are exceptions.

The amount of change isn't the same between every successive rock formation (Pictet). But if you compare formations separated by a significant amount of time, all the species will show some change.

Extinct Species Don't Reappear: Once a species disappears completely from the Earth, the exact same form never shows up again later. (Apparent exceptions, like M. Barrande's "colonies," are better explained as temporary migrations from other areas, as Lyell suggested).

How These Patterns Fit the Theory

These facts fit well with evolution by natural selection:

Slow, Uneven Change Expected: The theory doesn't predict any fixed rule making all life change together or at the same speed. Evolution must be slow. It usually affects only a few species at a time because variation happens independently in each species. Whether useful variations get preserved and accumulated depends on many complex factors: whether the right variations appear, how much interbreeding occurs, environmental changes, new species immigrating, and competition with other inhabitants. So, it makes sense that some species stay the same much longer than others, or change less. (Just like today, the land creatures of Madeira changed more than its sea creatures compared to Europe).

Why Faster Change on Land / in Higher Forms? Maybe because land environments are more complex and variable, and "higher," more complex organisms have more intricate relationships with their surroundings and each other, leading to stronger competition and faster adaptation.

Why All Species Eventually Change: If many inhabitants in an area improve over time, any species that doesn't improve will eventually be outcompeted and become extinct. So, over very long periods, all species must change or disappear.

Gaps Don't Mean New Creations: Because fossil-rich rocks mostly form during periods of sinking land (subsidence), with long blank intervals between them, the amount of change we see between consecutive formations won't be perfectly equal. Each formation isn't a new, complete creation story; it's just an occasional snapshot from a long, slowly changing drama.

Why Extinct Species Stay Extinct: Evolution works by modifying existing species. If a species goes extinct, its unique lineage is broken. Even if the exact same environmental conditions returned, a new species filling the same role would almost certainly inherit different traits from its different ancestor. It wouldn't be identical to the lost species. (Think pigeons: you might breed a new fantail-like bird, but you couldn't create an identical Fantail if the original rock pigeon ancestor was also extinct).

Groups of Species Follow the Same Rules

Genera (groups of related species) and families (groups of related genera) appear and disappear gradually, just like single species do. Once a group is gone, it never reappears. This fits the theory perfectly, because all species in a group are descendants of a common ancestor. Their existence is a continuous, branching chain of generations. (Lingula species, for example, represent an unbroken chain from the Silurian to today).

Gradual Increase and Decrease: While some groups seem to appear abruptly in the fossil record (explained last chapter by the record's imperfection), the general rule is a slow increase in the number of species within a group, reaching a peak, and then a gradual decline towards extinction. This matches the idea of evolution as a slow branching process like a tree growing from a single stem.

Extinction Explained by Natural Selection

The theory connects the origin of new species and the extinction of old ones.

The old idea of worldwide catastrophes wiping out all life periodically is largely abandoned.

Tertiary fossils show species and groups disappear gradually, one by one, place by place. (Sometimes extinction might be rapid, like if an isthmus breaks and lets in many new competitors).

Species and groups last for very different lengths of time. There's no fixed lifespan.

Extinction seems generally slower than production (the "thinning out" at the top is more gradual than the "thickening" at the bottom of the group's history). Sudden extinctions (like ammonites) might relate to gaps in the record or rapid replacements.

Extinction Isn't Mysterious: People often marvel at extinction, especially of large animals. When I found a fossil horse tooth with giant extinct mammals in South America, I was amazed, because modern horses thrive there wildly. But the fossil horse was a different, extinct species. Rarity is common for many species today, caused by unfavorable conditions we rarely understand. If conditions slowly worsen, a species becomes rarer, then extinct, its place taken by a competitor. It's like being surprised when a sick person dies after accepting sickness is common.

Unseen Checks Cause Extinction: We constantly forget that powerful, unseen forces check population growth. These same forces cause rarity and, ultimately, extinction. Surprise at Mastodon extinction ignores this; mere size doesn't guarantee survival (it might even hasten extinction due to food needs - Owen). Elephants today are held in check (by insects? - Falconer, Bruce). Insects/bats limit large mammals in parts of South America.

Competition Drives Extinction: Natural selection favors advantageous new forms, which outcompete and replace less-favored old forms. Improved domestic breeds constantly replace older, inferior ones (like Shorthorn cattle replaced earlier breeds "like some murderous pestilence"). Appearance of new and disappearance of old are linked. While more new species might arise than old ones go extinct sometimes, overall numbers haven't grown indefinitely, suggesting production and extinction balance out over later geological time.

Closest Relatives Suffer Most: Competition is usually fiercest between closely related forms (similar needs/structure). So, improved descendants generally exterminate their parent species. A new species presses hardest on its closest relatives (other species in the same genus), often driving them extinct. A successful new genus can supplant an older related genus. Sometimes a successful species invades the niche of a different group, causing extinction there too. The losers are often related because they share some inherited weakness.

Survival of Relicts: A few members of a declining group might survive for a very long time if they live in isolated places or have peculiar habits that protect them from severe competition (e.g., Trigonia shells in Australia; Ganoid fish in freshwater).

Conclusion: Extinction patterns fit well with natural selection. It's the expected result of competition and the production of improved forms.

Simultaneous Changes in Life Around the World?

The Observation: A striking fact is that life forms seem to change in parallel worldwide. The European Chalk formation, for example, can be identified in North and South America, Africa, and India – not by the rock type, but because the fossils, while not identical species, belong to the same distinctive families, genera, and sections, sometimes even sharing minor details. Fossils found above or below the Chalk in Europe also appear in the same order elsewhere. Similar parallelism seen in Paleozoic and Tertiary fossils across continents.

Marine vs. Land: This applies mainly to marine life. We lack data for land/freshwater parallelism. (South American extinct giant mammals wouldn't have been easily dated if found in Europe without context).

"Simultaneous" Is Approximate: This doesn't mean change happened in the same year or century. It's a large-scale geological synchrony. (Comparing modern Europe vs. Pleistocene Europe to the Southern Hemisphere shows the difficulty of precise correlation). But over vast time, later formations worldwide do share related fossils and lack older types, allowing broad correlation.

General Law Needed: This global parallelism can't be explained by local causes like changing currents or climates (which differ greatly). It points to a general law governing life everywhere (as noted by de Verneuil, d'Archiac, Barrande).

Natural Selection Explanation: Fits well. New species arise from dominant forms (common, widespread, already successful). Dominant forms have best chance of spreading further, giving rise to new dominant varieties/species in new regions. Diffusion is slow but happens over time. Dominant forms eventually prevail globally. As new, improved groups spread, they displace older, inferior groups (often related ones) worldwide. Result: Succession of forms tends to correspond globally in appearance and disappearance.

Effect of Intermittent Record: If formations in different regions weren't deposited at exactly the same time (due to oscillations of level), the general sequence of life would look parallel, but the species at apparently corresponding levels wouldn't be identical, reflecting slightly different amounts of time for evolution/migration. This might explain observed differences between fossils in supposedly equivalent strata in Europe and North America.

Relationships Between Extinct and Living Species

Classification: All life falls into a few grand classes (explained by descent from common ancestors).

Ancient Forms Differ Most: Older fossils generally differ more from living forms (more time for divergence).

Extinct Forms Fill Gaps: Extinct species help fill gaps between living genera, families, orders (Buckland). They are often intermediate or connecting links (Owen's "generalized forms"; Agassiz's "prophetic types"; Gaudry's Attica fossils linking living genera). Examples: Ruminants/Pachyderms linked; Horse/Hipparion; Typotherium; Sirenia/Halitherium; Whales/Zeuglodon/Carnivora; Birds/Archeopteryx/Reptiles/Compsognathus. Paleozoic groups less distinct than modern ones (Barrande).

Nature of Intermediates: Extinct forms usually not directly intermediate between two living forms, but link them through a circuitous path via common ancestors. Often intermediate by having fewer differing characters than modern descendants (groups closer in past).

Theory Explains Affinities: Descent with modification explains these patterns. Diagram (Chapter 4) illustrates: more recent forms generally differ more from ancient progenitor. Ancient forms stand closer to branching points, thus intermediate between later diverging groups. Imperfect record means we rarely find all links, but expect older groups to be somewhat less distinct, which is what we find.

Other Geological Patterns Explained

Intermediate Faunas: Fauna of any period generally intermediate between preceding and succeeding periods (allowing for extinction/migration). Example: Devonian fossils intermediate between Silurian/Carboniferous. (Not always exact due to unequal intervals, variable species duration - Mastodon example).

Consecutive Formations Closely Related: Fossils from consecutive formations much more related than from remote ones (Pictet). Not due to similar physical conditions (life changed globally under different climates). Explanation: Closely linked by generation. Imperfect record means we find closely allied "representative species," not perfect gradations. Evidence of slow change.

Advance in Organisation? Selection leads to improvement relative to conditions -> specialization increases -> higher organization. New forms beat old ones -> progress by "battle test." Most paleontologists agree organization has advanced overall, though hard to prove definitively.

Objections: Some simple forms persist unchanged (Brachiopods, Foraminifera). Response: No problem if simple conditions remain; no necessary law of progress for all lineages.

Difficulties: Hard to define/compare "highness" (sharks vs. bony fish? cuttlefish vs. bee?). Need compare all forms (high/low), relative numbers. Hopelessly complex comparing imperfectly known faunas. (NZ vs. GB life illustrates difficulty predicting "higher" standard).

Ancient Animals Resemble Embryos: (Agassiz). Geological succession parallels embryonic development. Theory Explanation: Variations occur later in life, inherited at corresponding age -> adult changes, embryo remains picture of past state. (Proof difficult, needs pre-Cambrian fossils).

Succession of Same Types in Same Areas: Remarkable law: Fossils resemble living forms in the same continent (Australia marsupials; S. America Edentata/armadillos; Owen extends to Old World; NZ birds; Madeira shells). Meaning: Not due to climate (differs between continents with similar types). Not immutable law (Europe had marsupials; distributions change). Theory Explanation: Inheritance! Inhabitants leave modified descendants in the same region. Differences between continents persist over time. Migration/competition eventually change patterns. (Note: Living sloth not direct degenerate descendant of giant Megatherium - giants extinct. But other fossils are close to living relatives, maybe ancestors. Evolution usually involves extinction of most species in a genus).

Summary of Geological Evidence and Theory

Geological Record Imperfect: Exploration limited; preservation selective; huge time gaps between/within formations; migration obscures local evolution. Explains why we don't find infinite fine transitional links, and why groups appear "suddenly." Rejecting record imperfection means rejecting theory.

Paleontological Laws Agree with Theory:

Slow, successive appearance of species.

Different rates of change.

Extinct species don't reappear.

Groups follow same rules (gradual rise/fall, extinction).

Extinction linked to new species production via competition.

Parallel succession of life worldwide (due to dominant forms spreading).

Affinities of extinct/living forms explained by branching descent.

Ancient forms often intermediate between later groups.

Consecutive formations have closely related fossils.

Organisation generally advances (though not universally or uniformly).

Ancient forms resemble modern embryos.

Succession of same types in same areas explained by inheritance.

Overall Conclusion: Despite the geological record's imperfection, its main facts strongly support the theory that species were produced by descent with modification (variation and natural selection – survival of the fittest), with old forms being replaced by new and improved ones over vast periods.

Where Life Lives: Understanding Geographical Distribution

Why do different plants and animals live in different parts of the world? This chapter explores the patterns of life across the globe and how they fit with the theory of evolution.

Fact 1: Climate Isn't the Whole Story

The first big thing we notice is that the types of plants and animals living in different regions cannot be completely explained by climate or other physical conditions alone. Almost everyone who studies this subject agrees.

America Example: The Americas have incredibly diverse environments – deserts, mountains, rainforests, grasslands, rivers, lakes – covering almost every temperature imaginable. You can find conditions very similar to those in the Old World (Europe, Asia, Africa). Yet, the native plants and animals of the Americas are fundamentally different from those of the Old World (except in the far north where the continents nearly touch).

Southern Hemisphere Example: Compare large areas in Australia, South Africa, and western South America with similar climates (between latitudes 25° and 35°). The environments are extremely alike, yet their native plants and animals are drastically different from each other.

Same Continent Example: Compare southern South America (below lat. 35°) with regions further north (above lat. 25°). The conditions are quite different, yet the life forms are much more closely related to each other than they are to life in Australia or Africa, even where the climate is nearly identical.

Sea Life: Similar patterns exist in the oceans.

Conclusion: Something other than just climate and physical conditions determines where species live.

Fact 2: Barriers Matter A Lot

The second big observation is that barriers preventing free movement (migration) are closely linked to the differences in life between regions.

Oceans: The huge difference between Old World and New World life shows the importance of oceans as barriers. Australia, Africa, and South America are also separated by vast oceans, and their life forms are very distinct.

Land Barriers: Even on the same continent, we see differences across major barriers like high mountain ranges, large deserts, or even big rivers. Because these barriers aren't as impassable or as permanent as oceans, the differences across them are smaller than between continents.

Sea Barriers: The Pacific coast and Atlantic/Caribbean coast of South America have very different marine life (though the Isthmus of Panama seems to have been open in the past, as about 30% of fish species are the same on both sides). The vast open Pacific Ocean acts as a barrier; the eastern Pacific islands have a completely different set of marine species than the American coasts. But traveling further west across the Pacific, where there are many islands (stepping stones) and no major barriers until Africa, we don't find sharp divisions in marine life; many species range across this huge area.

Conclusion: Barriers that prevent species from spreading are crucial in explaining why different regions have different life forms.

Fact 3: Life on the Same Continent is Related

The third major point is that plants and animals living on the same continent (or in the same sea) show a clear relationship or affinity to each other, even if the exact species change from place to place.

Every continent shows this. Traveling north to south, you see groups of related but distinct species replacing each other. Birds might have similar songs or nests.

South America Example: The southern plains have one species of Rhea (American ostrich); the northern plains have a different Rhea species – not an African Ostrich or Australian Emu. The rodents (agouti, bizcacha) resemble hares/rabbits in habits but have a distinct American structure. High mountains have an alpine bizcacha. Rivers have coypu and capybara (American-type rodents), not beavers or muskrats.

Islands: Islands near a continent have life that is clearly related to that continent, even if the island species are unique.

Fossils: Looking back in time, fossils found on a continent are related to the animals living there today.

Conclusion: There's a deep connection linking life across space and time within the same large region, regardless of physical conditions. What creates this connection?

The Explanation: Common Ancestry and Modification

The connection is inheritance from common ancestors.

Similarity: Organisms in the same region are related because they descended from common ancestors that lived in that region.

Dissimilarity: Differences between regions arise from modification through variation and natural selection after populations become separated. Different physical conditions might play a smaller role.

Degree of Difference: How different life is between regions depends on:

How effective barriers were at preventing migration.

How long ago separation occurred.

Which species managed to immigrate in the past.

How species interacted and competed, favoring different adaptations (the struggle for life is key).

Barriers are important because they stop migration. Time is important because modification is slow. Dominant species (already successful and widespread) have the best chance of spreading to new areas, where they face new conditions, vary further, and give rise to new related groups.

This idea of descent with modification explains why related species, genera, and even whole families are often found confined to the same geographical areas.

Species Originate in One Place ("Single Centres of Creation")

The theory implies that all species belonging to the same genus originated from a single ancestral species in one location.

Similarly, all individuals of the same species must have originated in one place first. It's unbelievable that identical complex organisms could be created independently at different spots on Earth.

The Question: Were species created at just one point, or multiple points? Understanding how species spread (dispersal) is difficult for many cases.

Argument for Single Origin: It's the simpler view and uses a known cause (reproduction and migration) rather than invoking miracles (multiple creations). Most species live in continuous areas; widely separated populations are seen as exceptional. Mammals, which can't easily cross oceans, show this clearly – no identical land mammals on distant, unconnected continents like Europe and Australia, even if climates are similar. Plants sometimes are identical because their seeds have better means of dispersal. Barriers explain why distinct groups exist. Natural groups (genera with closely related species) are usually confined to one region or have a continuous range. It would be strange if this pattern suddenly broke down at the species level.

Conclusion: It seems most probable that each species originated in a single area and then spread as far as its ability to migrate and survive allowed, given past and present conditions.

Difficult Cases: Yes, many cases are hard to explain – how did a species get from A to B? But past geographical and climate changes must have broken up formerly continuous ranges. We need to consider if the hard cases are numerous and problematic enough to abandon the single-origin idea, which is supported by general evidence. (Specific hard cases discussed later).

Migration + Modification: If we can show that migration likely occurred between two regions now having related but distinct species, it strongly supports the theory. Example: A volcanic island forms near a continent. It gets colonized by a few species from the mainland. Over time, their descendants become modified but remain clearly related to the mainland species. This is common and hard to explain by separate creation. (Matches Mr. Wallace's idea: species arise near pre-existing allied species).

(Note: This "single centre" idea differs from whether a species arose from a single pair or many individuals simultaneously. For interbreeding organisms, many individuals likely change together, influenced by selection on the whole group, not just descent from one pair like in asexual organisms or isolated varieties).

How Species Spread: Means of Dispersal

To understand distribution, we need to know how organisms move around.

Climate Change: Powerful influence. A region impassable now might have been easily crossed under a different past climate.

Changes in Land Level: Also crucial. Submerging an isthmus connects seas; land bridges connect continents or islands, allowing migration. We know great changes in land level have occurred.

Forbes's Idea (Extreme View): Edward Forbes suggested vast land bridges recently connected most continents and islands to explain distributions. Author's View: Disagrees with such enormous recent changes. Evidence supports oscillations, but not connecting everything recently. Arguments against Forbes: large differences in marine life across continents; local relationships between fossils and living forms; island life patterns; volcanic nature of oceanic islands (not continental remnants). Agrees: Former islands probably existed as stepping-stones and are now sunken (atolls). Knowing dispersal methods will help future speculation on past land connections.

Occasional ("Accidental") Means: Important for crossing barriers. Focus on plants:

Floating Seeds/Fruit: Experiments showed many seeds survive long immersion in seawater (e.g., 64/87 kinds survived 28 days). Drying helps some fruits/branches float much longer (e.g., hazelnut 90 days; asparagus 85 days). Estimate: Perhaps 10-14% of plant species could float ~900 miles on ocean currents and still germinate. Larger fruits floating longer helps species with restricted ranges.

Drift Timber: Reaches even remote islands. Carries stones (used for tools) and earth trapped in roots. Author germinated plants from earth in oak roots.

Floating Carcasses: Bird carcasses sometimes float; seeds in their crops can remain viable for weeks (e.g., peas germinated after 30 days in pigeon crop).

Birds Carrying Seeds: Highly effective. Birds blown far by storms. Hard seeds pass through digestive tracts unharmed (found germinable seeds in small bird droppings). Seeds in crops (undigested food storage) stay viable 12-18 hours -> bird blown 500 miles -> killed by hawk -> seeds scattered. Seeds in pellets disgorged by hawks/owls germinate. Fish eat seeds -> birds eat fish.

Insects Carrying Seeds: Locusts blown hundreds of miles offshore. Swarms visited Madeira. Belief in Natal: locust dung introduces seeds. Author tested dried locust pellets -> grew grass plants. A swarm could easily transport seeds to islands.

Mud on Birds' Feet/Beaks: Earth sometimes sticks. Found significant amounts on partridge/woodcock legs containing viable seeds (e.g., 82 plants from 6.5 oz earth on partridge leg!). Migrating birds arriving with dirt on feet must occasionally transport seeds.

Icebergs: Carry rocks, earth, even nests -> must transport seeds occasionally, especially during glacial periods (Lyell). Explains northern plants on Azores (where erratic boulders found).

Effectiveness: These occasional means, acting over tens of thousands of years, make wide dispersal likely. Transport is not purely accidental (currents, winds have patterns). Distances limited (seeds die in sea eventually, pass through birds quickly). Enough for crossing moderate seas, island hopping, but not usually between distant continents (explains distinct continental floras). Chance of successful colonization small for any single seed, BUT on a newly formed, poorly stocked island, chances much better. Occasional transport crucial over geological time while islands formed and before fully populated.

Distribution Explained by the Glacial Period

The Puzzle: Identical plants found on distant mountain tops (Alps, Pyrenees, N. Europe; White Mtns USA vs. Labrador vs. Europe) and in Arctic/Antarctic regions, separated by vast lowlands where they can't survive. Led early naturalists (Gmelin) to think of multiple creations.

The Key: Glacial Period (Ice Age): Agassiz highlighted its importance. Clear evidence (scored rocks, boulders, moraines) shows Europe and North America recently had arctic climate.

Forbes's Explanation:

Cold Advances: Arctic life moves south, temperate life moves further south (or dies if blocked). Mountain life descends to plains. At peak cold, arctic life covers central Europe/USA. (Circumpolar life fairly uniform).

Warmth Returns: Arctic life retreats north AND ascends mountains as snow melts. Temperate life follows northwards.

Result: Same species end up in Arctic regions and isolated on mountain tops far apart. Explains shared species US/Europe mountains. Explains mountain plants related to forms due north (migration path).

Modification on Mountains: Alpine species isolated since warmth returned -> exposed different conditions, competitors -> liable to modification -> some identical, some varieties, some distinct related species found on different ranges today. (Arctic forms migrated together, less disturbed, less modified).

What About Before the Ice Age? Needed similar temperate species across Old/New Worlds before glaciation started. Explanation: Pre-glacial (Pliocene) climate warmer -> life lived further north -> near Polar Circle, land almost continuous (Europe-Siberia-America) -> allowed intermigration -> uniformity.

Earlier, Warmer Periods: Extend back further (Older Pliocene?). Common circumpolar flora/fauna migrated south as climate cooled long before Ice Age -> descendants now modified in temperate Europe/US. Explains relationship (many allied/representative species, few identical) between Europe/N. America despite Atlantic. Land bridge existed then, now broken by cold. Once separated south of polar regions -> independent modification began long ago, leading to current differences.

Marine Life Parallel: Similar southern migration of marine life from uniform polar seas during warmer past explains related forms in now-sundered temperate seas (E/W North America; Mediterranean/Japan). Inexplicable by creation given different conditions.

The Glacial Period in the Southern Hemisphere & Croll's Theory

Evidence for glacial action also widespread in Southern Hemisphere (NZ, Australia, S. America). Seemed to imply simultaneous global cooling.

Mr. Croll's Theory: Glacial periods caused by changes in Earth's orbit eccentricity, affecting ocean currents. Key Idea: When one hemisphere has severe cold, the other is actually warmer with milder winters. Cold periods alternate between hemispheres (last great N. Hem. Glacial ~240k years ago). Older glacial periods likely occurred too.

This Explains Tropical Mountains & North/South Links:

During Northern Glacial: Arctic forms move south; vigorous Northern temperate forms invade equatorial lowlands. Southern hemisphere warmer -> equatorial forms move there.

Warming: Northern forms retreat north from equator; some strand on mountains. Southern forms return south.

During Southern Glacial: Southern temperate forms invade equator. Northern forms on mountains descend, mingle.

Warming: Southern forms retreat south, taking some northern forms with them; leave some southern forms on mountains.

Result: Explains identical/allied species in N/S temperate zones AND on tropical mountains (identical species possible; long isolation leads to representative species/varieties). Also explains distinct genera shared across these areas (from older glacial cycles).

North-to-South Migration Stronger: More northern forms migrated south than vice versa (Hooker, de Candolle). Reason: N. Hemisphere has more land -> larger populations -> more advanced/dominant through competition. When mingled at equator, northern forms won out, held mountain positions, spread south. Southern forms less successful invading north. (Analogy: European species easily naturalize S. Hemisphere; few southern species naturalize Europe). Mountains are "islands" on land; their inhabitants yielded to dominant northern forms.

Applies to Marine Life Too: Changed ocean currents during glacial periods allowed temperate species to reach equator -> some migrated to other hemisphere immediately (cool currents), others waited in deep cold water for opposite hemisphere's glacial period to allow further spread.

Remaining Difficulties

Still many unsolved distribution puzzles (exact migration routes; why specific species spread/modify; remote southern islands sharing plants - icebergs? distinct southern genera shared - ancient Antarctic flora dispersed before extinction?).

Conclusion on Glacial Periods

Croll's theory of alternating warmer/colder periods in opposite hemispheres, combined with slow modification, explains a vast number of facts about the distribution of identical and allied species worldwide – in temperate zones north and south, and on tropical mountains. Life flowed north and south, stronger from the larger, more competitive north. It left stranded populations on mountains, like high-water marks, recording past distributions.

Life in Freshwater and on Islands

How do plants and animals spread to isolated places like lakes, rivers, and distant islands? Let's look at the patterns we see and what they tell us about evolution.

Freshwater Life: Surprisingly Wide Ranges

You might think freshwater creatures wouldn't travel far. Lakes and river systems are isolated by land. The sea seems like an even bigger barrier.

The Reality: Surprisingly, the opposite is true. Many freshwater species (fish, insects, shells, plants) are found over enormous areas. Related freshwater species often exist across the entire world.

Personal Example: When I first collected freshwater life in Brazil, I was amazed how similar the insects and shells were to those in Britain, while the surrounding land animals were completely different.

How Do Freshwater Species Spread So Far?

The answer likely lies in their ability to move short distances frequently – from pond to pond or stream to stream within their own country. This ability makes wider dispersal much easier.

Fish: This group presents challenges. It used to be thought the same freshwater fish species never lived on different continents. But Dr. Günther found one species (Galaxias attenuatus) living in Tasmania, New Zealand, the Falkland Islands, and South America! This amazing case might be explained by dispersal from Antarctica during a past warmer period. (This fish genus can also cross considerable ocean distances somehow). Even within one continent, freshwater fish ranges can seem random – neighboring rivers might share some species but have completely different ones too.

How they travel:

Accidents: Fish dropped by whirlwinds; eggs surviving out of water for some time.

Geography Changes: Land level changes connecting rivers (likely main cause); floods connecting rivers temporarily. (Mountain ranges blocking river connections lead to different fish on each side).

Time: Some freshwater fish are ancient forms, allowing lots of time for migration during vast geographical changes. Fish species also seem to endure for long periods (Günther).

Sea Travel?: Saltwater fish can sometimes adapt to fresh water. Many freshwater groups have some marine relatives. A marine relative could travel along coastlines and potentially adapt to freshwater in a distant land.

Freshwater Shells: Also have huge ranges. How do they cross land and sea barriers? Their eggs (and adults) are killed by salt water, and birds probably don't carry the eggs often.

My Observations:

Ducks emerging from ponds sometimes carry tiny duck-weed plants on their backs. I accidentally transferred freshwater shells between aquariums this way.

More Effective: Tiny, newly hatched snails crawled onto a duck's feet I submerged in an aquarium. They clung so tightly they couldn't be shaken off and survived in damp air for 12-20 hours. A duck or heron could easily fly hundreds of miles in that time, land on a distant pond, and transport the snails.

Water beetles can also carry small shells (Ancylus found stuck to Dytiscus). A water beetle flew onto my ship 45 miles from land – who knows how much farther it could be blown?

Freshwater Plants: Many marsh and freshwater plants have enormous ranges, found on continents and remote islands. Aquatic members of mostly land-based plant families often have particularly wide ranges (De Candolle).

Dispersal by Birds: Wading birds visit muddy pond edges. Mud sticks to their feet and beaks. These birds migrate vast distances and visit remote islands. They fly to freshwater haunts upon landing. Pond mud is loaded with seeds! (My experiment: 3 tablespoons of mud from a pond edge yielded 537 plants of many kinds!). It seems inevitable that water birds transport freshwater plant seeds over huge distances. They might transport small animal eggs this way too.

Other Means: Fish eat seeds. Herons eat fish. Herons fly far, then reject undigested seeds in pellets hours later – seeds can still germinate (even large water-lily seeds found in heron stomach).

Why Freshwater Colonization Might Be Easier

Empty Niches: A newly formed pond or stream (e.g., on a rising island) is empty. A single seed or egg arriving has a good chance of success.

Less Competition: Even established ponds have fewer species compared to land areas. Competition is likely less intense, making it easier for a newcomer to establish itself.

Slower Evolution: Many freshwater organisms are relatively simple ("low in the scale"). Simpler organisms seem to evolve more slowly, giving them more time to migrate before changing significantly.

Past Connections: Many freshwater forms likely had continuous ranges in the past, which were later broken up by extinction in intermediate areas.

Main Reason: Wide dispersal seems mostly due to transport of seeds and eggs by animals, especially wide-ranging water birds.

Oceanic Islands: Natural Laboratories

Now let's look at oceanic islands – islands far from continents, usually formed by volcanoes. Their inhabitants provide crucial evidence. (I don't believe these islands were recently connected to continents by vast land bridges, as some have suggested).

Key Features of Island Life:

Few Species: Islands have far fewer species than similar-sized areas on continents (De Candolle, Wollaston). New Zealand has only 960 flowering plants despite its size and diverse habitats, compared to thousands in similar areas of Australia or South Africa. Even small areas in Britain have comparable numbers. Barren islands like Ascension had almost none initially.

Implication: If species were specially created, not enough were created for islands. Humans have introduced species and stocked islands much more effectively than nature did originally! Introduced species often drive natives extinct (St. Helena).

Many Endemic Species: A very high percentage of island species are endemic – found nowhere else in the world (e.g., Madeira land shells, Galapagos birds).

Explanation: Expected by theory. Colonists arrive rarely, over long intervals. They face new conditions and new competitors. This makes them highly likely to undergo modification, often producing unique groups of descendants.

Not Uniform: Doesn't mean all groups become endemic. Depends on arrival patterns (did they come as a group or one by one?) and whether later immigrants arrive and interbreed, checking modification. (Galapagos: most land birds endemic, few sea birds endemic – sea birds arrive easily. Bermuda: no endemic land birds – many visitors from N. America arrive frequently. Madeira: 1 endemic bird, many endemic land shells, no endemic sea shells – reflects different dispersal abilities).

Absence of Whole Groups: Oceanic islands often lack entire classes of animals. Their ecological roles are sometimes filled by other groups.

Galapagos: Reptiles were dominant where mammals would be.

New Zealand: Giant flightless birds were dominant land animals instead of mammals.

Explanation: Often blamed on physical conditions, but this is doubtful. Ease of immigration seems just as, or more, important.

Strange Life Forms:

Islands without mammals sometimes have plants with hooked seeds (hooks normally used for catching onto fur). Likely the ancestor had hooks for dispersal by mainland mammals; the plant reached the island by other means, became endemic, and kept the now-useless hooks.

Islands often have tree-like plants belonging to groups that are only herbs elsewhere. Trees disperse poorly. An herbaceous plant colonizing an island might gain an advantage by growing taller than other herbs -> selection favors tree-like form.

Missing Groups: Frogs and Land Mammals

Batrachians (Frogs, Toads, Newts): Never found on truly remote oceanic islands (Bory St. Vincent). (Exceptions like New Zealand are likely not true oceanic islands). Not due to conditions – introduced frogs thrive on islands like Madeira. Reason: They and their spawn are killed by seawater, making ocean crossing extremely difficult. (Why not created there? Hard to explain).

Terrestrial Mammals: Absent from islands more than ~300 miles from continents (excluding human introductions). (Falklands fox is closest exception, but islands close to mainland via submarine bank, received icebergs). Small islands can support mammals if close to continents. Reason: Cannot cross wide seas.

Aerial Mammals (Bats): Found on almost every island, even remote ones, often as unique endemic species (New Zealand, Pacific islands, Mauritius). Explanation: Bats can fly across oceans. They wander far. Colonized islands -> modified over time. Absence of other land mammals explained by dispersal limits.

Sea Depth Matters for Island Mammals

There's a link between the depth of the sea separating islands/mainlands and how closely related their mammals are (Earl, Wallace).

Islands on shallow banks near continents have similar or identical mammals (Britain/Europe; Australia nearby islands).

Islands separated by deep water have related but distinct mammals (West Indies - distinct species/genera from America).

Explanation: Shallow seas more likely indicate recent land connections, allowing less time for modification after separation. Deeper channels imply longer isolation, more modification. Inexplicable by independent creation.

Overall Island Pattern: Colonization and Modification

Island life features (few species, many endemic, missing groups, strange forms) fit better with occasional colonization over long periods, followed by modification, than with past land bridges (which would imply more uniform immigration and less modification).

Remaining Difficulty: How did some things, like land shells (killed by salt water), reach remote islands? Unknown means must exist (tiny young on bird feet? hibernating shells in driftwood? – experiments show some survive seawater).

The Strongest Pattern: Island Life Resembles Nearest Mainland

This is the most crucial fact.

Galapagos Example: 500-600 miles off South America. Everything bears the stamp of the American continent (birds, reptiles, insects, plants). Why? Conditions are not particularly similar. Compare Galapagos to Cape Verde Islands (similar volcanic islands, climate) – Cape Verde life related to Africa!

Explanation: Independent creation offers no answer. Descent with modification is obvious: islands receive colonists mainly from the nearest source. Colonists then become modified, but inheritance betrays their origin.

Universal Rule: Endemic productions of islands related to nearest continent or large island. Exceptions often explainable (Kerguelen plants related to America via iceberg drift?). New Zealand related to Australia (nearest), also South America (explained via Antarctic dispersal?). SW Australia/Cape Hope plant link still puzzling.

Island Hopping: Modification Within Archipelagos

The same pattern occurs within island groups (archipelagos).

Galapagos: Each island often has distinct, unique species. BUT these species are much more closely related to each other than to mainland forms. Expected: immigrants came from same source, spread between islands.

The Puzzle: Why did immigrants modify differently on islands so close and physically similar? Answer: Physical conditions aren't everything. The community of other species is crucial. Different colonists might reach different islands initially. An immigrant arriving faces different competitors/enemies on each island -> natural selection favors different variations. (Some species might spread unchanged).

Why Didn't Island Species Spread to All Islands? Galapagos islands more isolated than they look (deep channels, strong currents, rare gales). Also, don't assume easy invasion. If islands already occupied by well-adapted species, newcomer might not succeed even if closely related (Madeira/Porto Santo shell example: species don't cross despite stone transport, but European species invade both). Preoccupation is important.

The General Principle: Colonization + Modification Applies Everywhere

The pattern seen on islands (relation to source + modification) applies broadly:

Mountain Tops: Alpine species related to surrounding lowland species (colonized as mountains rose).

Lakes and Marshes: Inhabitants related to nearby land dwellers (unless wide dispersal possible).

Caves: Blind animals related to surface animals nearby.

Rule: Where two distant regions share related/representative species, they usually share some identical species too. Where related species occur, expect doubtful forms (varieties) showing modification steps.

Wide-Ranging Genera Have Wide-Ranging Species

Genera found worldwide often contain at least some species with very wide ranges (birds, bats, cats, dogs, insects, freshwater life).

Supports Theory: If all species in genus descend from one ancestor, expect some descendants to inherit capacity/opportunity for wide dispersal. (Not all species range widely; depends on adaptation and competition).

Lower Organisms Range More Widely

Law (observed by de Candolle for plants): Lower/simpler groups range more widely.

Explanation: Lower organisms often ancient -> more time to disperse. Change slower -> remain same species longer while dispersing. Seeds/eggs often tiny -> better transport.

Conclusion on Distribution Patterns

All these relationships (lower forms wider ranges; wide genera have wide species; local life related to surroundings; island life related to nearest source; closer relations within archipelagos) are inexplicable by independent creation. They are perfectly understandable as results of colonization from the nearest or easiest source, followed by adaptation and modification.

Summary of Chapters on Geographical Distribution (Previous and Present)

Single Origin Belief: Believing all individuals of a species descended from common parents in one area is plausible, despite dispersal difficulties (consider past climate/land changes, dispersal means, intermediate extinctions). Supported by importance of barriers and distribution patterns of related groups. Believing distinct species in a genus spread from one source also plausible (allow for slow change, long time).

Glacial Period: Played huge role, mixing N/S hemisphere life, stranding forms on mountains.

Dispersal Means: Various occasional methods (esp. for freshwater) allow crossing barriers over time.

Theory Explains Grand Facts: If migration from single source + modification accepted, main facts of distribution understandable:

Importance of barriers (forming provinces).

Concentration of related species in same areas.

Mysterious links between inhabitants of different habitats within same continent, and to extinct forms.

Why similar conditions have different life (depends on colonization history, competition).

Island patterns (few species, many endemic, missing groups, peculiar forms).

Island relations (to mainland, within archipelago).

Presence of identical species where representative species exist.

Parallelism in Time and Space: Laws governing life succession (geology) similar to laws governing distribution now (geography) (Forbes). Species endurance continuous; groups have max development points; shared trifling features; different change rates; lowly forms change less. Intelligible: Whether looking at change over time or space, connected by common descent; same variation laws; same natural selection mechanism.

Relationships: Classification, Anatomy, Embryos, and Vestiges

This chapter looks at how living things are grouped, how their bodies are built, how they develop as embryos, and why they sometimes have useless parts. All these areas provide strong clues about how life evolved.

Classification: Grouping Life

From the earliest known history of life, organisms have resembled each other to different degrees. This allows us to classify them into groups nested within larger groups (like species within genera, genera within families, families within orders, and so on).

Not Arbitrary: This isn't like grouping stars into random constellations. There's a real pattern.

Not Based on Lifestyle: It's not simply based on where animals live or what they eat. Members of the same group often have very different habits.

Reflects Evolution: Remember how dominant species (common, widespread) tend to vary most, producing new varieties that become new species? These new species inherit traits, leading to related groups that grow and branch out over time (divergence). Extinction removes intermediate forms. The result is exactly the pattern we see: groups within groups.

Diagram Example: The branching diagram shown earlier illustrates this. All descendants from ancestor (A) form a large group (like a Class). Branches that split off later form smaller related groups (Orders, Families, Genera).

What is the "Natural System" of Classification?

Scientists try to arrange life according to a "Natural System." What does this mean?

Simple View: Just grouping similar things together and separating dissimilar things. It's a useful way to summarize information.

Deeper Meaning? Many scientists feel it's more than just similarity. Some thought it revealed a "Creator's plan," but that's vague. Old sayings like "the genus gives the characters" hint at a deeper connection.

The Author's View: That deeper connection, the hidden bond scientists have been searching for, is community of descent – shared ancestry through inheritance. Natural classification is genealogy, mapping out the family tree of life as best we can, even though it's obscured by evolutionary changes.

How Classification Works (and Why It's Hard)

Let's look at the rules scientists follow and the problems they face. These make sense if classification is based on descent.

Important vs. Unimportant Traits:

Traits related directly to lifestyle (like body shape for swimming in whales vs. fish) are considered less important for finding deep relationships. These are called adaptive or analogical characters.

Traits less related to specific habits are often more important (like reproductive organs - Owen). In plants, reproductive parts (flower, seed, embryo) are more important than vegetative parts (leaves, stems).

Morphological details (structure patterns) with no clear function are often highly valued if they are consistent across many related species. Why? Because natural selection doesn't strongly act on useless traits, they tend to remain unchanged and reflect shared ancestry.

Value Changes: An organ's importance for survival doesn't dictate its value in classification. The same organ (like antennae in insects) can be crucial for classifying one group but minor in another, even if its physiological function is similar (Robert Brown quote).

Rudimentary Organs: Useless or shrunken organs (like tiny teeth in whale embryos, leg bone remnants in ruminants) are often very valuable for classification because they clearly indicate ancestry.

Trifling Characters: Small, seemingly minor details (nostril position, jaw angle, wing folding, skin covering like hair/feathers) can be very important if consistent across diverse species. They likely point to inheritance from a common ancestor.

Aggregate Characters: A combination of many characters, even minor ones, is much more reliable than any single character. That's why classification based on one trait always fails. It explains why we group organisms based on an overall "feel" of resemblance, even if we can't define every single shared point (Linnæus).

Practical Method: Scientists use characters that are consistent within a group and different between groups. The more consistent and widespread a character, the higher its value.

Embryonic Characters: These are considered highly important (Milne Edwards, Agassiz), often more so than adult features. Plant classification relies heavily on embryo structure. (We'll see why shortly – they reveal ancestry).

Chains of Affinity: Sometimes, groups at opposite ends of a spectrum have almost nothing in common, but are linked by a chain of intermediate species. This allows us to place them in the same larger group (like crustaceans).

Geography: Sometimes used, especially for closely related forms (birds, insects). Species in one isolated region likely share a recent ancestor.

Arbitrary Ranks: The value given to ranks like "family" or "order" seems somewhat arbitrary. Groups are often raised or lowered in rank as new intermediate species are discovered, showing the connections are more gradual than the ranks imply.

Descent Explains Classification

All these rules and difficulties make perfect sense if the Natural System is based on descent with modification:

True Affinity is Ancestry: Characters showing real relationships are those inherited from a common ancestor.

Why Adaptive Traits are Misleading (Between Groups): Similar lifestyles can cause unrelated organisms to evolve similar features independently (analogy). These hide, rather than reveal, their true separate ancestries (e.g., whale shape vs. fish shape).

Why Adaptive Traits are Useful (Within Groups): Within a related group (like the whale family), shared adaptive traits (like body shape) do indicate true affinity because they were likely inherited from their common ancestor who first adapted that way.

Why Trifling/Rudimentary/Embryonic Traits Matter: They are less likely to have been recently modified by adaptation to specific habits. Therefore, they better reflect the underlying inherited pattern from ancestors.

Why Aggregate Characters are Reliable: It's unlikely that many independent, minor traits would evolve identically by chance in unrelated groups. A combination points strongly to shared inheritance.

Why Physiological Importance Varies: An organ important for life might be unchanged from an ancestor (high classificatory value) or heavily modified differently in various descendants (less value for grouping all together, more value for defining subgroups).

Classification is Genealogical: Like a family tree or pedigree. The arrangement shows relationships. The amount of difference acquired since divergence determines ranks (species, genus, family, etc.). (Language analogy: A family tree is the best way to classify languages, showing relationships and origins, even if some changed more than others).

Classifying Varieties

The same principles apply when classifying varieties of one species. We group sub-varieties under varieties, under the species. We use the most constant characters (like horns in cattle, not sheep - Marshall). A true pedigree would be the ideal classification, reflecting inheritance. (Tumbler pigeons grouped by tumbling habit, but short-faced variety kept with them due to blood relationship/other traits, despite losing the habit).

Naturalists Already Use Descent Unconsciously

Scientists always use descent without thinking about it:

Grouping males, females, young (larvae), alternate generations, monsters, and varieties under the same species, despite huge differences, because they share descent.

Proposal: Extend this principle consciously. Group species into genera, genera into families, etc., based on inferred community of descent, traced through similarities (especially non-adaptive ones).

Analogical Resemblances (Convergent Evolution)

Definition: Similarities between unrelated organisms due to adaptation to similar lifestyles, not common ancestry.

Examples: Dugong/whale vs. fish (body shape); mouse vs. shrew; mouse vs. marsupial Antechinus (active movement in dense vegetation); homopteran insect classed as moth; pig breeds; turnip stems; dog vs. Thylacine jaws (general shape for tearing flesh, but fundamental tooth differences).

More Amazing Examples: Electric organs in different fish; luminous organs in different insects; complex pollen structures in distant orchids and asclepiads. These look similar functionally but differ fundamentally in structure -> evolved independently. Analogous variation (shared tendency to vary similarly) might help selection produce similar outcomes.

Numerical Parallelism: Sometimes similar numbers of subgroups seen in distinct classes (sevens, fives, etc.). Might reflect adaptation to similar diverse niches, partly forced by classifiers valuing symmetry.

Mimicry: Butterflies imitating other distasteful species (Bates). Remarkable resemblance for protection, not shared habits. Explained by natural selection favoring variations that improve the resemblance, helping the rare "mimic" avoid predators that avoid the common, distasteful "model."

Extinction Defines Groups

Extinction has played a huge role in separating groups and making them distinct.

Wider gaps between major classes (like birds vs. other vertebrates) exist because many ancient connecting forms are lost.

Where less extinction occurred (like crustaceans), groups are linked by long, complex chains of related forms.

Extinction defines the boundaries, but doesn't create the groups (which arise from branching descent). If everything that ever lived reappeared, defining groups would be impossible, but a natural arrangement based on descent would still be possible, like branches on a tree. We could still identify "type" forms representing each cluster.

Classification Summary

Natural selection (via struggle, extinction, divergence) explains the universal pattern of life grouped within groups. Descent is the hidden bond. This view explains classification rules: why we value non-adaptive, rudimentary, embryonic, or aggregated characters, and disregard analogy between distant groups. It explains complex, radiating affinities and how extinction creates distinct groups. Natural classification is genealogy. Attempting to map this "Tree of Life" gives classification a clear goal. (Haeckel's phylogeny work mentioned as example).

Morphology: The Science of Body Plans

Definition: Study of the fundamental structure and arrangement of parts in organisms. Includes "Unity of Type."

Unity of Type: Organisms in the same class share a fundamental structural plan, regardless of lifestyle differences. "Soul" of natural history.

Homologous Structures: Parts in different species built on the same underlying pattern, inherited from a common ancestor, even if modified for vastly different functions.

Examples: Bones in human hand (grasping), mole foot (digging), horse leg (running), porpoise paddle (swimming), bat wing (flying) – same bones, same relative positions. Kangaroo hind feet (hopping vs. koala grasping vs. bandicoot digging) – all share unique fused-toe structure (contrast with American opossum).

Relative Position: Key to homology (Geoffroy St. Hilaire). Bones never transposed.

Insect Mouths: Huge variety (proboscis, tongue, jaws) all modifications of same basic parts (lips, mandibles, maxillae). Same for crustacean mouths/limbs, flower parts (modified leaves).

Cannot Explain by Utility: Hopeless to explain this shared pattern just by function or purpose (Owen admitted). Creation view just says "Creator planned it that way" (not scientific).

Theory Explains: Descent with modification explains homology simply. Selection modifies parts for new functions but tends not to alter the underlying pattern or transpose parts. If ancestor had limbs of general pattern -> descendants retain pattern modified for hands, wings, paddles. If ancestor insect mouth simple -> descendants modify parts diversely. (Pattern can get obscured by reduction/fusion/doubling of parts).

Serial Homology: Repeated Parts Within One Individual

Comparing different parts in the same individual.

Examples: Skull bones homologous to vertebrae? Crab jaws homologous to legs? Flower parts homologous to leaves? Embryos show parts starting alike then becoming different. Monsters show transformations possible.

Inexplicable by Creation: Why skull made of many bones like vertebrae (even in birds)? Why bat wing/leg same bones? Why crab complex mouth <-> fewer legs? Why flower parts same pattern?

Theory Explanation: Segmentation/symmetry origin obscure. BUT repetition of parts common in simple organisms (ancestors likely had many vertebrae, segments, leaves). Repeated parts vary easily (number/form). Provided raw material for adaptation to different roles. Inheritance preserves underlying resemblance. Variations similar early on -> serial homology maintained unless obscured. (Molluscs have less repetition -> fewer serial homologies).

Lankester's Distinction: Homogenous (similar due to common ancestor) vs. Homoplastic (similar not due to common ancestor - includes analogy, serial/lateral homology). Important refinement.

Metamorphosed Organs: Language like "skull from vertebrae" metaphorical usually, but descent makes it almost literally true (both from simpler common element).

Embryology: Development Reveals History

Metamorphosis: Insect changes seem abrupt but involve many gradual hidden stages (Chlöeon molts 20+ times). Crustaceans show huge changes. Alternate generations (coralline/jellyfish) amazing cycles. Larva producing larva (Wagner fly) links to parthenogenesis -> maybe explains asexual reproduction at early stage?

Key Embryological Facts:

Embryo usually differs from adult (but not always – cuttlefish, land snails).

Parts alike early become different later.

Embryos of distinct species (same class) often very similar (Von Baer: can't tell early mammal/bird/lizard apart). Larvae too (crustaceans, barnacles). Resemblance sometimes lasts late (young bird plumage, lion cub spots, plant first leaves).

Embryo structures often useless then/later (gill slits/arteries in mammal/bird embryo).

Active larvae perfectly adapted to conditions (Lubbock). Similarity sometimes obscured by larval adaptations. Larvae can differ more than adults.

Development usually progresses (butterfly > caterpillar), sometimes regresses (parasites, complemental male barnacles).

Explanation: Based on two principles:

Variations generally appear not very early in life.

Inherited at a corresponding not early age.

Why young resemble each other: Variations affecting adults inherited late -> embryo little modified -> resembles ancestor more closely. Use/disuse effects also act late, inherited late. (Pigeon breed example: young much more similar than adults, except short-faced tumbler).

Why young sometimes resemble adult: If variations appear/inherited earlier. Explains groups without metamorphosis (land snails etc.). Likely favored if young need same adaptations as parents early on (e.g., adapting to land/freshwater - Müller).

Why larvae differ from adults: If young profit from different habits -> selection modifies larva -> metamorphosis results. Larval stages acquired via adaptation.

Embryo as Picture of Ancestor: Embryo shows, more or less, adult or larval state of ancient progenitor. Community of embryonic structure reveals community of descent. Dissimilarity doesn't disprove (stages maybe suppressed/modified). Explains why ancient fossils resemble modern embryos. (Law may be true but unprovable if record incomplete).

Rudimentary Organs: Useless But Informative

Definition: Organs imperfect, useless, bearing stamp of inutility. Extremely common (male nipples; snake lung lobe; bird 'bastard wing'; fetal whale teeth; calf unerupted teeth).

Evidence of Origin: Easily identified (beetle wing stubs under fused covers). Sometimes retain potential (male milk; extra cow teats). Can vary between individuals (petal size). Crossing reveals nature (rudimentary pistil enlarges in hybrid). Useless for one function, used for another (swimbladder lung; male flower pistil style brushes pollen). Useless organ serving no purpose (salamander tadpole gills in mother).

Distinguish Rudimentary vs. Nascent: Nascent = developing towards future use (rare, usually supplanted). Rudimentary = remnant of past use. (Penguin wing maybe reduced/modified, not nascent; Apteryx wing rudimentary. Lepidosiren limbs likely remnants. Ornithorhynchus glands nascent? Cirripede frena nascent gills?).

Characteristics: Highly variable (no longer checked by selection). Degree of reduction varies between related species (female moth wings). Can be totally aborted (missing parts expected by homology - fifth stamen). Useful for tracing homologies. Often detectable embryo, disappear later. Relatively larger in embryo than adult.

Meaning: Astonishing! Useless organs exist. Creation explanations ("symmetry," "complete nature's scheme," excretion) fail/inconsistent (why only some snakes retain leg bones? Excreting lime via fetal teeth?). Stump nails useless. Descent Explanation: Simple. Originates from disuse (like domestic examples). Disuse main agent (cave eyes, island wings). Selection helps reduce if injurious (Madeira beetles). Modification for new purpose possible. Variability expected (no selection check). Inherited at mature age -> relatively larger in embryo.

Remaining Difficulty: How final reduction/obliteration happens after functionless? Disuse ends. Maybe innate tendency to shrink > grow? Economy of growth helps early stages, not for tiny vestiges.

Conclusion: Rudimentary organs = record of past state, kept by inheritance. Useful classification clue (genealogy). Existence expected under descent, strange difficulty for creation. Like useless letters in spelling clue word origin.

Chapter Summary (Synthesis)

Classification: Grouping reflects genealogy (common descent + modification). Explains rules/difficulties, value of characters, radiating affinities, extinction defining groups. Natural System is genealogical.

Morphology: Unity of type (homologous structures) in class, serial homology in individual -> explained by inheritance from common ancestor with modifications.

Embryology: Embryo/adult differences; early similarity -> later divergence; larval adaptations; retrograde development; resemblance ancient forms -> explained by variations arising/inherited late in life -> embryo picture of ancestor.

Rudimentary Organs: Uselessness, variability, embryonic state -> explained by inheritance + disuse (aided selection/economy). Clue to ancestry/classification.

Final Conclusion: These four fields (Classification, Morphology, Embryology, Rudimentary Organs) provide strong, independent lines of evidence. They seem to plainly proclaim that all species descended, within their groups, from common parents, modified during descent. Author would adopt this view based on this evidence alone, even without other arguments.

Wrapping Up: Objections, Evidence, and Looking Forward

This whole book has been one long argument for evolution by natural selection. Let's briefly summarize the main points, objections, and conclusions.

Reviewing the Challenges

I don't deny that there are many serious objections to the theory that species change over time through variation and natural selection. I've tried to address them honestly.

Complex Organs and Instincts: It seems incredibly hard to believe that complex organs like the eye, or amazing instincts like a bee building honeycomb, were perfected through tiny, gradual variations, each useful to the owner, rather than by some intelligent design. However, this difficulty isn't impossible to overcome if we accept three things:

All body parts and instincts do show at least small individual differences.

There is a struggle for existence that leads to helpful differences being preserved.

Gradual steps in the perfection of any organ likely existed, each step being useful in its own way.
I think these three points are undeniable. It's hard to imagine all the steps for many structures, especially in groups with many extinct relatives, but nature shows us so many strange gradations that we should be cautious about saying something couldn't have evolved gradually. Special difficulties, like the sterile worker ants, have been tackled.

Sterility of Species Crosses: The fact that different species are usually sterile when crossed, while varieties are usually fertile, seems like a major difference. However, as summarized in Chapter 9, this sterility doesn't seem like a special creation to keep species separate. It looks more like an accidental side effect of differences in the reproductive systems, similar to why some trees can be grafted onto others and some cannot. The different results of reciprocal crosses (A x B vs. B x A) and the facts about dimorphic/trimorphic plants support this view.

Fertility of Varieties: While generally true, it's not universal that varieties are perfectly fertile when crossed (Gärtner and Kölreuter found exceptions). Domestication seems to remove sterility barriers rather than create them, likely because domestic animals have become used to changing conditions.

The Conditions-Crossing Parallel: There's a striking parallel: slight changes in conditions OR slight crosses (between varieties) seem beneficial, boosting vigor and fertility. Large changes in conditions OR large crosses (between species) often cause sterility. This suggests a deep link related to the fundamental nature of life and how systems respond to disturbance. If we could fully explain why wild animals often become sterile in captivity, we might understand why hybrids are often sterile.

Geographical Distribution Difficulties: It's often hard to imagine how species spread to distant, isolated places from a single origin point. But we must remember:

Species can persist unchanged for vast periods, allowing immense time for migration.

Extinction in intermediate areas can explain broken ranges.

We are still ignorant about past climate and geography changes, and all the ways organisms can occasionally be transported.

Modification is slow, allowing time for dispersal.

Missing Intermediate Links (Living): We don't see chaos today because selection exterminates intermediates, competition creates boundaries, and intermediate zones are often unfavorable for hybrid varieties.

Missing Intermediate Links (Fossils): This remains the most obvious objection. Why aren't rocks full of intermediate fossils? Why do groups seem to appear suddenly? Why no fossils before the Cambrian? Answer: The geological record is incredibly imperfect.

Few organisms fossilize well.

Only a fraction of the world is explored.

Collections are tiny compared to past life.

Parent forms aren't directly intermediate.

Links wouldn't be recognized without the full chain.

Formations have huge time gaps between and within them.

Modification happens relatively quickly compared to periods of stability.

The earliest rocks might be deeply buried or transformed beyond recognition.

We don't know the true age of the Earth or when life began with certainty.

Conclusion: If we accept the record is extremely poor, the lack of infinite links and the apparent sudden appearances are understandable. Geology does show slow, graduated change over long timescales (fossils in consecutive formations are closely related).

Summary of Evidence Supporting the Theory

Now, let's look at the arguments for descent with modification through natural selection.

Lessons from Domestication:

Changed conditions cause variability.

Variation follows complex laws (correlation, use/disuse, etc.).

Humans select variations (methodically or unconsciously) to create breeds.

Selection accumulates tiny differences with large results.

Domestic breeds often resemble natural species, blurring the lines.

Natural Selection:

Principles effective in domestication should act in nature.

Survival of the fittest is a powerful, ever-acting force.

Struggle for existence is inevitable due to high reproduction rates.

Small advantages determine survival/reproduction. Competition is fiercest between closest relatives.

Sexual selection adds another layer (male competition/charms).

Variation in Nature:

Nature varies (individual differences, distinct varieties).

No clear line between variation levels (individual -> variety -> sub-species -> species).

Power of Accumulation:

If life varies (even slightly), why shouldn't useful variations be preserved and accumulated by natural selection over long ages?

This power, scrutinizing every detail and favoring the good, seems capable of producing complex adaptations. The theory seems highly probable just based on this.

Explanatory Power: The theory explains many large classes of facts:

Classification: Makes sense of the blurry line between species and varieties. Explains why dominant species in large genera have more varieties and resemble varieties themselves. Explains the hierarchical grouping (groups within groups) as a result of divergence and extinction – the Natural System is a genealogy.

Divergence: Explains how small differences become large ones, leading to distinct species and groups filling diverse ecological niches.

"Natura non facit saltum": Explains why nature proceeds by small steps, not sudden leaps.

Diverse Means for Same End: Explains why different organisms evolve different solutions for the same problem (inheritance + adapting existing structures).

Anomalous Habits/Structures: Explains woodpeckers on the ground, geese that don't swim, etc. (adaptation to new niches).

Beauty: Partly explained by sexual selection (male ornaments/songs) and attracting pollinators/seed dispersers (flower/fruit colors).

Imperfection: Explains why adaptations are relative to competitors, not absolutely perfect, and why some traits seem non-optimal (bee sting).

Laws of Variation: Parallelism between variety/species variation; effects of use/disuse, correlation; reversions (striped horses).

Patterns of Variability: Explains why specific characters, unusual parts, and secondary sexual characters are more variable.

Instincts: Graduated nature, mistakes, similarity in related species, origin via selection/habit fit the theory.

Hybrids/Mongrels: Similarities (except fertility) explained if species are just well-marked varieties.

Geological Succession: Slow appearance, different rates, extinction, group patterns, intermediate fossils, advance in organization, embryo resemblance, succession of types in same area – all align with descent.

Why Was Belief in Unchanging Species So Common?

If the evidence is strong, why did almost all top scientists until recently believe species were immutable (unchanging)?

Variation exists, amount not proven limited, no clear species/variety line, sterility not absolute test.

Main Causes:

Short Timescale: Belief the world was young made evolution seem impossible.

Assumed Perfect Record: Now know time vast, wrongly assume geological record good enough to show intermediates easily if they existed.

Difficulty Imagining Gradual Change: We struggle to grasp vast time and the cumulative effect of tiny changes over generations (like geology). Easy hide ignorance using phrases like "plan of creation." Weighing unexplained difficulties more than explained facts leads to rejection.

The Future of Natural History

Acceptance: Confident theory will be accepted by young, impartial naturalists. Expressing conviction helps overcome prejudice. (Notes shift: Few believed evolution when book first published; now almost all naturalists accept it, though some still prefer sudden, unexplained jumps).

Impact on Classification: Systematists can continue, but freed from doubt about species "reality." Focus shifts from defining undiscoverable "essence" of species to deciding if forms distinct enough for name, valuing actual amount of difference more. Treat species like genera (useful groupings based on descent). Classification becomes genealogy – mapping the Tree of Life. Rules simpler with clear goal (tracing descent). Rudimentary organs, aberrant ("living fossil") species, embryology become key tools.

Impact on Other Fields: Geography gains power tracing migrations using geology/climate change. Geology benefits understanding record imperfection, gauging time via organic change. Psychology gets foundation (gradual acquirement mental powers - Spencer). Origin of humans illuminated.

General Interest: Natural history becomes far more interesting when viewing organisms not as incomprehensible creations, but products of long history, structures/instincts as sum of useful adaptations (like inventions). Opens fields studying variation causes, use/disuse, conditions effects. Domestic productions become vital study subjects.

How Far Does the Theory Extend?

Question difficult; arguments weaker for more distinct forms.

Within Classes: Strong evidence (affinities, fossils, homology, embryology, rudiments) supports theory embracing all members of same great class (e.g., all vertebrates from one ancestor). Animals maybe from <= 4-5 progenitors, plants <= equal/lesser.

All Life from One Ancestor? Analogy leads further: maybe all animals and plants from one prototype. (Analogy risky). Shared chemistry, cells, growth laws, susceptibility suggest deep connection. Lowest forms intermediate animal/vegetable (Asa Gray). Seems credible both kingdoms evolved from simple intermediate -> all earthly life from one primordial form. (Inference based analogy; okay if not accepted. Maybe many forms started, few survived - Lewes). But evidence strong for single progenitor within each kingdom.

Concluding Thoughts

Some eminent authors satisfied with independent creation. To me, accords better with Creator's laws that life/extinction due to secondary causes (like individual birth/death).

View beings as lineal descendants ancient forms -> ennobles them.

Future: No living species stays unchanged; few leave descendants long-term (extinction norm). Common, widespread species in dominant groups will prevail, produce new dominant species.

Ordinary generation never broken; no global cataclysm -> secure future. Natural selection works for good -> progress towards perfection.

Final Metaphor: Contemplate tangled bank (plants, birds, insects, worms) -> complex, interdependent forms all produced by simple laws acting around us: Growth, Inheritance, Variability (conditions, use/disuse), High Reproduction -> Struggle for Life -> Natural Selection -> Divergence, Extinction. From war of nature -> highest object conceivable (higher animals). Grandeur: life breathed into few forms/one -> from simple beginning, endless forms most beautiful and wonderful have been, and are being, evolved, while planet follows physical laws.