Evolution and Natural Selection

The idea of evolution was known to some of the Greek philosophers. By the time of Aristotle, speculation had suggested that more perfect types had not only followed less perfect ones but actually had developed from them. But all this was guessing; no real evidence was forthcoming. When, in modern times, the idea of evolution was revived, it appeared in the writings of the philosophers-Bacon, Descartes, Leibniz and Kant. Herbert Spencer was preaching a full evolutionary doctrine in the years just before Darwin's book was published, while most naturalists would have none of it. Nevertheless a few biologists ran counter to the prevailing view, and pointed to such facts as the essential unity of structure in all warm-blooded animals.

The first complete theory was that of Lamarck (1744-1829), who thought that modifications due to environment, if constant and lasting, would be inherited and produce a new type. Though no evidence for such inheritance was available, the theory gave a working hypothesis for naturalists to use, and many of the social and philanthropic efforts of the nineteenth century were framed on the tacit assumption that acquired improvements would be inherited.

But the man whose book gave both Darwin and Wallace the clue was the Reverend Robert Malthus (1766-1834), sometime curate of Albury in Surrey. The English people were increasing rapidly, and Malthus argued that the human race tends to outrun its means of subsistence unless the redundant individuals are eliminated. This may not always be true, but Darwin writes:

In October 1838, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on, from long continued observation of the habits of animals and plants, it at once struck me that, under these circumstances, favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species. Here then I had a theory by which to work.

Darwin spent twenty years collecting countless facts and making experiments on breeding and variation in plants and animals. By 1844 he had convinced himself that species are not immutable, but worked on to get further evidence. On 18 June 1858 he received from Alfred Russell Wallace a paper written in Ternate, in the space of three days after reading Malthus's book. Darwin saw at once that Wallace had hit upon the essence of his own theory. Lyell and Hooker arranged with the Linnaean Society to read on July 1st 1858 Wallace's paper together with a letter from Darwin and an abstract of his theory written in 1844, Then Darwin wrote out an account of his labours, and on 24th November 1859 published his great book The Origin of Species.

In any race of plants or animals, the individuals differ from each other in innate qualities. Darwin offered no explanation of these variations, but merely accepted their existence. When the pressure of numbers or the competition for mates is great, any variation in structure which is of use in the struggle has 'survival value', and gives its possessor an improved chance of prolonging life and leaving offspring. That variation therefore tends to spread through the race by the elimination of those who do not possess it, and a new variety or even species may be established. As Huxley said, this idea was wholly unknown till 1858. Huxley said the book was like a flash of lightning in the darkness. He wrote

It did the immense service of freeing us from the dilemma - Refuse to accept the Creation hypothesis, and what have you to propose that can be accepted by any cautious reasoner? In 1857 I had no answer ready, and I do not think anyone else had. A year later we reproached ourselves with dullness for being perplexed with such an enquiry. My reflection when I first made myself master of the central idea of the Origin was 'How extremely stupid not to have thought of that!'

The hypothesis of natural selection may not be a complete explanation, but it led to a greater thing than itself-an acceptance of the theory of organic evolution, which the years have but confirmed. Yet at first some naturalists joined the opposition. To the many, who were unable to judge the biological evidence, the effect of the theory of evolution seemed incredible as well as devastating, to run counter to common sense and to overwhelm all philosophic and religious landmarks. Even educated man, choosing between the Book of Genesis and the Origin of Species, proclaimed with Disraeli that he was 'on the side of the Angels'.

Darwin himself took a modest view. While thinking that natural selection was the chief cause of evolution, he did not exclude Lamarck's idea that characters acquired by long use or disuse might be inherited, though no evidence seemed to be forthcoming. But about 1890 Weismann drew a sharp distinction between the body (or soma) and the germ cells which it contains. Somatic cells can only reproduce cells like themselves, but germ cells give rise not only to the germ cells of a new individual but to all the many types of cell in his body. Germ cells descend from germ cells in a pure line of germ plasm, but somatic cells trace their origin to germ cells. From this point of view, the body of each individual is an unimportant by-product of his parents' germ cells. The body dies, leaving no offspring, but the germ plasms show an unbroken continuity. The products of the germ cells are not likely to be affected by changes in the body. So Weismann's doctrine offered an explanation of the apparent noninheritance of acquired characters.

The supporters of pure Darwinism came to regard the minute variations as enough to explain natural selection and natural selection enough to explain evolution. But animal breeders and horticulturalists knew that sudden large mutations occur, especially after crossing, and that new varieties might be established at once. Then in 1900 forgotten work by Mendel was rediscovered and a new chapter opened.

In 1869 Darwin's cousin, Francis Galton, applied these principles to mental qualities. By searching books of reference, Galton examined the inheritance of ability. For instance, he found that the chance of the son of a judge showing great ability was about 500 times as high as that of a man taken at random, and for the judge's father it was nearly as much. While no prediction can be made about individuals, on the average of large numbers, the inheritance of ability is certain.

(From Chapter VIII of A Shorter History of Science by Sir W. C. Dampier.)