Darwinism, a name often erroneously applied to the theory of descent, a theory which Darwin in no wise originated, though he transformed it from the unverified hypothesis which it was in the hands of Lamarck (q.v.), Geoffroy St. Hilaire and Robert Chambers, into a well-authenticated and reasonable explanation. This transformation was accomplished by the theory of natural selection, and, though Darwin himself admitted that he had looked somewhat too exclusively upon natural selection as the agent of specific change, it is the theory of natural selection that is rightly termed Darwinism. "Species," he says, "have been modified, during a long course of descent. This has been effected chiefly through the natural selection of numerous successive, slight, favourable variations: aided in an important manner by the inherited effects of the use and disuse of parts, and in an unimportant manner by the direct action of external conditions, and by variations which seem to us in our ignorance to arise spontaneously." The biological laws upon which he bases his arguments are, in his own words, "Growth with reproduction; inheritance which is almost implied by reproduction; variability from the indirect and direct action of the conditions of life, and from use and disuse; a ratio of increase so high as to lead to a struggle for life, and as a consequence to natural selection, entailing divergence of character and the extinction of less improved forms." These laws have been otherwise expressed as amounting to - (1) the admitted facts of inherited likeness accompanied by individual variation; (2) the tendency, pointed out by Malthus, of all organisms to increase far beyond the space or subsistence available for them, while land, sea, and oxygen are limited in amount; (3) the necessarily resultant struggle for existence keenest among the closest allies; and (4) the inevitable natural selection, or survival of the fittest.
Darwin, it will be observed, makes little endeavour to explain the origin of variations. His subject is the origin of species or of specific divergence, the selection of certain variations and their isolation by the elimination of intermediate forms. He postulates spontaneous variation in various directions and a vast expanse of time in which slight variations may accumulate and compete in the battle for life. Mr. Wallace, who arrived at the theory of natural selection simultaneously with Darwin, has shown that many initial variations are by no means slight, so that natural selection may have acted rather more rapidly; and that, whatever action external conditions, the use and disuse of organs, or sexual selection may have, they will all be subordinated to, or regulated by, natural selection. Darwin substantiated his views by evidence drawn from embryology, and from the existence of rudimentary (now called" vestigial") organs; and answered in detail objections drawn from the alleged perfection of the organs of the senses, the alleged inerrancy of instinct, the alleged sterility of hybrids, and the absence, as far as our knowledge went in 1859, of the "missing-links," which, but for the imperfection of the geological record, we might expect to find as fossils.
Some of the effects of his theory upon biology have been to direct more minute attention to slight variations in both wild and domesticated forms; to make us look for the uses to its possessor of every detail of structure, and so to study habits more intelligently; to exhibit fossils to us as a connected series, and to get rid of all the old anxieties as to whether a form was or was not a "good species," i.e. a distinct creation. The sciences of embryology, comparative psychology, anthropology, and sociology, have received from Darwin's theory or from his rehabilitation of the general theory of descent an impulse all but tantamount to creation.
Recently some naturalists have revived Lamarck's view as to the importance of the use or disuse as modifying the form or structure of organs, whilst others altogether deny the inheritance of newly acquired characters.