He then promises four chapters on `the most apparent and gravest difficulties of the theory', prominent among these being to understand how a simple organism or organ can change into a highly complex one - another nod to Paley. The introduction ends with a flourish: I can entertain no doubt ... that the view which most naturalists entertain, and which I formerly entertained - namely, that each species has been independently created - is erroneous. I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendent of some other and generally extinct species ... Furthermore, I am convinced that Natural Selection has been the main but not exclusive means of modification.
In essence, Darwin's theory of natural selection, which soon became known as evolution,[1] is straightforward. Most people think they
[1] The term was around in Victorian times, as a phenomenon but not a specific mechanism. Darwin didn't use it in Origin, nor in the later The Descent of Man. However, the final word in Origin is 'evolved'. understand it, but its simplicity is deceptive, and its subtleties are easily underestimated. Many of the standard criticisms of evolutionary theory stem from common misunderstandings, not from what the theory actually proposes. The ongoing scientific debate about details is often misrepresented as disagreement with the general outline, which is an error based on too simple-minded a view of how science develops and what `knowledge' is.
Briefly, Darwin's theory goes like this.
1. Organisms, even those in the same species, are variable. Some are bigger than others, or bolder than others, or prettier than others.
2. This variability is to some extent hereditary, passed on to offspring.
3. Unchecked population growth would quickly exhaust the capacity of the planet, so something checks it: competition for limited resources.
4. Therefore as time passes, the organisms that do survive long enough to breed will be modified in ways that improve their chance of surviving to breed, a process called natural selection.
5. Ongoing slow changes can lead, in the long run, to big differences.
6. The long run has been very long indeed - hundreds of millions of years, maybe more. So by now those differences can have become huge.
It's relatively simple to put these six ingredients together and deduce that new species can arise without divine intervention - provided we can justify each ingredient.
Even though different species seem to stay pretty much the same - think lions, tigers, elephants, hippos, whatever - it is actually rather obvious that, in general, species are not fixed for all time. The changes are relatively slow, which is why we don't notice them. But they do happen. We've already seen that in Darwin's finches, evolutionary changes can be and have been observed on a timescale of years, and in bacteria they occur on a timescale of days.
The most obvious evidence for the variability of species, in Darwin's day and ours, was the domestication of animals - sheep, cows, pigs, chickens, dogs, cats ...
... and pigeons. Darwin was rather knowledgeable about pigeons, he belonged to two London pigeon clubs. Every pigeon-fancier knows that by selectively breeding particular combinations of male and female pigeons, it is possible to produce `varieties' of pigeons with particular characteristics. `The diversity of the breeds is something astonishing,' says Darwin in the first chapter of Origin. The English carrier pigeon has a wide mouth, large nostrils, elongated eyelids, a long beak. The short-faced tumbler has a short stubby beak like a finch. The common tumbler flies high up in a tightly knit flock, and has an odd habit of falling about in the sky, whence its name. The runt (despite its name) is huge, with a long beak and large feet. The barb is like the carrier but with a short, broad beak. The pouter has an inflatable crop and can puff out its chest. The turbit has a short beak and a line of reversed feathers on its chest. The Jacobin has so many reversed feathers on its neck that they form a hood. Then there are the trumpeter, laugher, fantail ... These are not separate species: they can interbreed, to produce viable `hybrids' - cross-breeds.
The enormous variety of dogs is so well known that we don't even need to mention examples. It's not that the dog species is exceptionally malleable, just that dog-breeders have been unusually active and imaginative. There is a dog for every purpose that a dog can carry out. Again, they're all dogs, not new (albeit related) species. They can mostly (barring really big size differences) interbreed, and artificial insemination can take care of mere size. Dog sperm plus dog egg makes fertile dog zygote, and, eventually, dog - independently of breed. That's why pedigree pooches need a pedigree, to guarantee that their parentage is pure. If the different varieties of dog were different species, that wouldn't be necessary.
In modem times, it has become clear that cats are just as malleable, but the cat-breeders have only just got going on exotic cats. The same goes for cows, pigs, goats, sheep ... and what about flowers? The number of varieties of garden flowers is immense.
By avoiding the creation of hybrids, the breeder can maintain the individual varieties over many generations. Pouter pigeons breed with pouters to produce (a substantial proportion of) pouters. Carriers mated with carriers produce (mostly) carriers. The underlying genetics, about which Darwin and his contemporaries knew nothing, is complicated enough that apparent hybrids can sometimes arise from what seems to be pure stock, just as two brown-eyed parents can nonetheless have a blue-eyed child. So pigeon-breeders have to eliminate the hybrids.
The existence of these cross-bred varieties does not, of itself, explain how new species can arise of their own accord. Varieties are not species; moreover, the guiding hand of the breeder is evident. But varieties do make it clear that there must be plenty of variability within a species. In fact, the variability is so great that one can readily imagine selective breeding leading to entirely new species, given enough time. And the avoidance of hybrids can maintain varieties from one generation to the next, so their characters (biologese for the features that distinguish them) are heritable (biologese for `able to be passed from one generation to the next'). So Darwin has his first ingredient: heritable variability.
The next ingredient was easier (though still controversial in some quarters). It was time. Oodles and oodles of time, the Deep Time of geologists. Not a few thousand years, but millions, tens of millions ... billions, in fact, though that was further than the Victorians were willing to go. Deep Time, as we've previously observed, is contrary to the biblical chronology of Bishop Ussher, which is why the idea remains controversial among certain Christian fundamentalists, who have bizarrely chosen to fight their corner on the weakest of grounds, completely needlessly. Deep Time is supported by so much evidence that a truly committed fundamentalist has to believe that his God is deliberately trying to fool him. Worse, if we can't trust the evidence of our own eyes, then we can't trust the apparent element of `design' in living creatures either. We can't trust anything.
Lyell realised that the age of the Earth must be many millions of years, when he looked at sedimentary rocks. These are rocks like limestone or sandstone which form in layers, and have been deposited either underwater, as muddy sediments, or in deserts, as accumulating sand. (Independent evidence for these processes comes from the fossils found in such rocks.) By studying the rate at which modern sediments accumulate, and comparing that with the thickness of known beds of sedimentary rock, Lyell could estimate the time it had taken for the layers of rock to be deposited. Something in the range 1000-10,000 years would produce a layer about a metre thick. But the chalk cliffs of the south coast, around Dover, are hundreds of metres thick. So that's several hundred thousand years of deposition, and we've only dealt with one of the numerous layers of rock that make up the geological column - the historical sequence of different rocks.