Having just re-read (I had sort of flicked through them both before, years ago) Darwin’s On the Origin of Species and The Descent of Man, I have a much clearer idea of what he was actually saying. There is a tendency, even among experts, to conflate ‘Darwinism’ with the theory of evolution in general, and even with the whole field of genetics. This is useful shorthand, but it can hide the differences in a way that becomes important in a subject which is often discussed more from the perspective of religion and politics than from that of science. So let’s look at what Darwin actually did.
A Disclaimer: I remind readers once more that I am not a geneticist or evolutionary biologist, any more than I am a linguistician, historian, palaeoanthropologist, indologist or any of the other things I like to chat about so breezily here. I was once a mathematician, but that was a long time ago. I state the facts as far as I know them, and quote from people who seem to know what they’re talking about the rest of the time.
Anyhow, Darwin didn’t invent the theory of evolution. Plenty of people had proposed such a theory before him. What he did that was revolutionary, and the reason his name is so closely associated with the concept, was to collate thousands of detailed and precise observations on form and behaviour of life of all kinds from all over the world, including skeletal structure, internal organs, body covering, colour, predatory, defensive and sexual behaviour, dimorphism, and history. Much of this information was his own, gathered in the course of long and difficult voyages by sea to just about everywhere- he was in Australia, Tierra del Fuego, the Pacific Islands, the Phillipines and the Galapagos, as well as many more places closer to home- and where the data are second hand he is careful to establish the competence of his informant and the extent to which the information can be trusted.
His inescapable conclusion from analysing all of these data was that all life forms change with time, that many have once been very different from how we see them now, that some have derived from an earlier common form, and that this occurs in nature because of pressure of two kinds- forms better adapted to the specific circumstances they live in tend to live longer and leave more offspring, and forms better suited to defeating rival males and attracting females tend also to leave more offspring.
That organisms can change over the generations was already well known, from experiments on short-lived insects which can be observed through hundreds of generations over a period of years, and from breeding of domesticated animals, detailed records of which exist going back hundreds or even thousands of years, but that the same thing happened in nature, without the intervention of man, was the subject of considerable doubt. Darwin effectively dispelled that doubt. And by applying the same observations to primates, he showed the very high probability that man shared a common ancestor with the extant apes, and these with the primates in general.
However, although he described clearly the behavioural and environmental pressures which bring about change in organisms, what he did not do was identify the biological mechanism by which this process occurs. He had no idea what a gene was, as they were not discovered until many decades later. Nor did he postulate anything remotely like them. He speaks of passing on individual characteristics from parent to offspring, but the nearest he comes to conceiving of genes is when he speaks of ‘gemmules’, which are poorly defined and are in any case a very long way from being genes. The theory of gemmules is not his, and when he mentions them he doesn’t sound very happy about the idea. It’s just that he had nothing else.
He also had very limited information about man. With the exception of the Neanderthals, no early hominid remains had been clearly identified at that time. All he had to go on was morphology. Nevertheless, in placing the origin of modern man in Africa he was almost certainly right.
He was wrong about certain things. I’m sure there were errors, minor ones, in some of the observations. It would be extraordinary if there were not. But his main error is perhaps the residual Lamarckism. He often refers to the inheritance of acquired characteristics, along with chance variations at birth. He had no reason at that time to imagine that only variations brought about in a particular way could be transmitted to offspring, again because he had no conception of the mechanisms involved.
The real genius of Darwin was not his foresight, nor his conceptual brilliance, but his ability to gather and analyse data on such an enormous scale, and with such attention to detail.