Indeed, as he also pointed out, diversification is itself a biological advantage, since it enables a given area to support a greater bulk of living matter, and in general makes it possible for life to exploit the resources of the environment more fully. Though later-evolving groups are more highly organized, he rightly argued that we should not expect all groups and types of organisms to evolve in the direction of higher organization. Thus single-celled forms, through the very fact of their small size and rapid reproduction, fill a certain natural niche more successfully than larger multicellular creatures can do.

He further showed that natural selection, by its nature, could never cause or promote the evolution of a character which was primarily of advantage to another species. This is a negative generalization as important in its way as the impossibility of perpetual motion is in physical science.

He anticipated modem evolutionary genetics by deducing that large species (with an abundance of individual members) and large genera containing many species will be more variable than small ones, and more likely to produce new species in the course of evolution.

Of course, his views have often had to be modified in detail. This is especially true when he is discussing heredity and variation, since in his day the mechanisms of genetics and mutation were completely unknown.

There was a period, from about 1895 to about 1925, when Darwinism came in for a great deal of criticism, sometimes violent, from many of the leading biologists of the time. They questioned the very notion that characters such as cryptic or warning coloration are advantageous or adaptive, and indeed were inclined to dismiss the whole idea of adaptation contemptuously as mere teleological speculation. The Lamarckians and the Vitalists rejected the idea of natural selection as too materialistic and as giving insufficient weight to will and effort and other psychological forces. The early Mendelians, fascinated by the discovery of genetic units (gene-differences) with large effects (such as hornless-ness in cattle, or albinism in many vertebrates), wanted to make mutation responsible for positive evolutionary change, and assigned to natural selection only the essentially negative function of getting rid of harmful variants. Many of them rejected the idea of slow biological improvement and gradual evolution in favour of stepwise mutational changes of large extent. The position was finally complicated by the fact that the biometricians, preoccupied with the gradual variation so often found in nature, wanted to deny the importance of Mendelism, and confused the heritable variations of character produced by changes in the genes with the nonheritable ones produced by changes in the environment.

Eventually, however, these various contradictions were reconciled. Lamarckian and vitalistic explanations were ruled out when it was shown that acquired characters, whether impressed by the environment or resulting from use or individual effort, were never inherited. The advance of genetics showed that large mutations were rarer and of far less biological importance than those of small extent, and that, apparently, continuous evolutionary change could be, and often was, brought about by the accumulation of numerous small discontinuous mutations under the guidance of natural selection. And finally R. A. Fisher in 1930 made it clear that the fact of heredity being particulate—dependent on distinct self-reproducing units or genes, each of which could mutate into new self-reproducing forms—and the further fact that most mutants are recessive, at once got rid of the major difficulties that beset Darwin, who accepted the current view of blending heredity—the view that characters and the entities that determined them were commingled into a single blend when crossed. This would imply that any new character would be progressively diluted by crossing in each generation, and would make its establishment in the stock difficult. But a particulate genetic mechanism in which most mutants are recessive makes it possible for new mutants to be stored indefinitely in the constitution, and for new combinations of new and old genes to be formed, ready to be utilized by selection when conditions are favourable. Natural selection was seen, not as involving the sharp alternatives of life or death, but as the result of the differential survival of variants; and it was established that even slight advantages, of one-half of one percent or less, could have important evolutionary effects.

With this, Darwinism took on a new lease of life. Neo-Darwinism, as we may call the modern theory of gradual transformation operated by natural selection acting on a Mendelian genetic outfit of self-reproducing and self-varying genes, is fully accepted by the great majority of students of evolution. Darwin would have rejoiced to see how, even in the light of our enormously increased knowledge, it (and it alone) can account for the varied and often puzzling facts of evolution—the different types and degrees of adaptation; the (geologically speaking) rapid transformation of some types side by side with the unchanged persistence of others; the coexistence of lower and higher forms; the succession of constantly more improved types; extinction ; the facts of geographical distribution; the evolution of insect societies based on elaborate instincts. Today, a century after the publication of the Origin, Darwin’s great discovery, the universal principle of natural selection, is firmly and finally established as the sole agency of major evolutionary change.

In the Origin, Darwin had already adumbrated many ideas which he or others afterward followed out in more detail. Thus he mentions the existence of plants with two or three types of flower, like primroses, and explains it as a means of avoiding self-fertilization and inbreeding, a subject which he amplified in two books published in 1876 and 1877. He explained how psychological characters like instincts and emotions could and would be evolved by natural means, though it was not until 1872 that, with his remarkable book The Expression of the Emotions in Man and Animals, he initiated the comparative study of animal behaviour, or ethology as it is now called. By his constant emphasis on the complicated web of interrelations between different organisms in a given habitat or region he laid the foundations for the new and flourishing branch of science now known as ecology.

In the Origin he briefly outlines his theory of sexual selection, which he later (in 1871) developed at great length in his big book The Descent of Man. He regarded sexual selection as a subsidiary mechanism of evolution, needed to account for the development of male secondary sexual characters, notably weapons like stags’ antlers or exaggerated plumes and conspicuous displays like those of many male birds. Though this theory has been bitterly attacked (often by those without adequate knowledge of the facts), and has had to be modified in various particulars, it provides another example of Darwin’s originality and insight. He correctly deduced that such characters, though irrelevant in the general struggle for existence or in competition with other species, would be of advantage in what he called the struggle for reproduction, which must exist between differently endowed males. Selection here is therefore sexual, or as the modem formulation more correctly puts it, intrasexual—between members of the same sex. It is indeed the best example of intraspecific selection, the result of competition between members of the same species; and the resultant characters (for instance the fantastically exaggerated wings of the Argus pheasant) may even be a disadvantage in the struggle for existence.

Further, though most courtship-display characters are directed to raise the female’s readiness to perform the act of mating rather than towards her choice of one male rather than another as mate, it remains true that, as Darwin suggested, they exert their effect via the sense organs, mind and brain of another individual—in this case the female. They are among the most obvious examples of such allaesthetic characters, as we now style them. Thus Darwin had hit on the important principle that selection could be of different types, acting in different and sometimes contradictory ways, and through different operational channels.

Though Darwin was cautious not to put forward unsupported conclusions, once he had established the validity of his ideas to his own satisfaction he did not hesitate to draw the fullest implications from them—though even here, his modesty was always in evidence.