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What Darwin said

 There are two misunderstandings about Darwin. First, that he invented the idea of evolution and, second, that he put forward a theory of the origin of life. He did neither. Evolution – the idea that existing forms of plants and animals had evolved from earlier forms – existed before Darwin. What Darwin did was to provide a convincing theory as to how the different species of plants and animals had come about. He said nothing about the origin of life, only that an original life-form must have existed (however it might have come into existence). Darwin's theory was that evolution came about through natural selection. In fact the term “Darwinian” is more appropriately applied to the theory of natural selection than to the theory of evolution.

 Before him, some people including his grandfather, Erasmus Darwin, had realised that existing species must have evolved from previously existing species. A study of the classification of life-forms by Linnaeus in the 18th century into Kingdoms, Orders, Genera and Species, based on the physical similarities between them, suggested this. But earlier evolutionists could not offer a convincing explanation as to how this came about.

  Perhaps the most famous pre-Darwinian theory was that of Lamarck, who argued that new species came about through characteristics acquired during lifetime being transmitted to descendants – the theory of the inheritance of acquired characteristics. Thus, for instance, giraffes evolved as a previous animal stretched its neck from generation to generation. The theory is not true, as can be seen from the Jews. They've been circumcising their sons for thousands of years but no Jew has ever been born without a foreskin. Actually, it wasn't really such a laughable theory that could be dismissed in this way.

  Lamarck was a serious scientist and it was a valid hypothesis. In fact it was the main one going till Darwin came up with his theory 150 years ago. Engels in his The Part Played by Labour in the Transition from Ape to Man accepted it when he was talking about the diet of humans (and foxes) leading to them changing. Even Darwin, although a severe critic of Lamarck, was prepared to backtrack a little and conceded that it might have played a minor role in the evolution of species. Opponents of the “you can’t change human nature” argument were also attracted by it.

 Lamarck was on the right track about the evolution of giraffes. It did come about gradually as he supposed, but not directly. Giraffes evolved over generations as animals with longer and longer necks survived better. Similarly, with those who saw that human behaviour did, and could, change: humans can inherit acquired characteristics but not biologically, only culturally, through learning.
 Farmers – and pigeon fanciers (with whom Darwin associated in his research) – had known long before Darwin that they could create new forms of existing plants and animals by selective breeding. Darwin's argument was that the same process had happened in nature over a long period of time – with nature doing the selecting – and that this had resulted in all the various life-forms that did exist (and had existed) as evolved forms of the original life-form. A separate "species" (as opposed to a "race" or a subspecies which was what farmers and pigeon-fanciers were creating) came into being when its members were not able to breed with the life-form from which they evolved or with other life-forms which had arisen from it. This was Darwin's theory of the origin of species.

  Another misunderstanding about Darwin arises from the phrase “the survival of the fittest”, which he did use. It is often seen as meaning the survival of the physically fittest, i.e. of the strongest, but the word “fittest” was not being used in this sense in this context. It meant rather the “aptest”, or the “most fitted” to a particular environment, and it meant that relatively more of their offspring survived than did those of the less apt.
 Darwin did not know, any more than the farmers and pigeon-fanciers, how selection worked. He suspected that there must be some cause of the minute changes that nature worked upon but couldn't explain how they arose. The solution was left to later scientists who, following up research by an Austrian monk, Gregor Mendel, who was experimenting at the same time as Darwin with sweet peas, came up with the gene as the unit of inheritance, with the small changes being caused by inexact copies of genes being made in certain individuals. The combination of Darwin's theory of natural selection and the gene theory is known as the "modern synthesis" and is what is defended by present-day defenders of evolution such as Richard Dawkins and other popular science writers, even though some of them have been tempted to turn “Darwinism” into a general theory of the “survival of the aptest” in all fields and not just to the evolution of species of living things.