We do not propose to expound the Darwinian theory for the hundredth time; a knowledge of it must be taken for granted. We need only briefly call to mind the characteristic features and catchwords of the theory as Darwin founded it, which have also been the starting points of subsequent modifications and controversies.

All living creatures are bound together in genetic solidarity. Everything has evolved through endless deviations, gradations, and differentiations, but at the same time by a perfectly continuous process. Variation continually produced a crop of heterogeneous novelties. The struggle for existence sifted these out. Heredity [pg 092] fixed and established them. Without method or plan variations continue to occur (indefinite variations). They manifest themselves in all manner of minute changes (“fluctuating” variations). Every part, every function of an organism may be subject individually to variation and selection. The world is strictly governed by what is useful. The whole organisation as well as the individual organs and functions bear the stamp of utility, at least, they must bear it if the theory is correct. In the general continuity the transitions are always easy; there are no fundamentally distinct “types,” architectural plans, or groups of forms. Where gaps yawn the intermediate links have gone amissing. There is no fundamental difference between genus, species, and variety. Even the most complicated organ such as the eye, the most puzzling function such as the instinct of the bee, may be explained as the outcome of many more primitive stages.

The chief evidences of the theory of descent are to be found in homologies, in the correspondences of organs and functions, as revealed by comparative anatomy and physiology, in the recapitulation revealed by embryology, in the structure of parasites, in rudimentary organs and reversions to earlier stages, in the distribution of animals and plants, and in the possibility of still transforming, at least to a slight extent, one species into another, by experimental breeding.

Transformation and differentiation go on in nature as a vast, ceaseless, but blind process of selection. In [pg 093] artificial selection evolution is secured by choosing the most fit for breeding purposes; so it is secured in natural selection by the favouring and survival of those forms which are the most fit among the many unfit or less fit, which happened to be exposed to the struggle for existence, that is, to the competition for the means of subsistence, to the struggle with enemies, to hostile environment, and to dangers of every kind. The adaptation thus brought about is of a purely “passive” kind. The variations arise fortuitously out of the organism, and present themselves for selection in the struggle for existence; they are not actively acquired by means of the struggle. The secondary factors of evolution recognised are: correlation in the growth and in the development of parts, the origin of new characters through use, their disappearance through disuse (Lamarck), the transmission of characters thus acquired, the influence of environment and sexual selection.5

The Darwinian theory, the interpretation of the teleological in the animate world by means of the theory of descent based upon natural selection, entered like a ferment into the scientific thought-movement, and in a space of forty years it has itself passed through a series of stages, differentiations, and transformations which have in part resulted in the present state of the theory, and have in part anticipated it. These are represented [pg 094] by the names of workers belonging to a generation which has for the most part already passed away: Darwin's collaborateurs, such as Alfred Russel Wallace, who independently and simultaneously expounded the theory of natural selection, Haeckel and Fritz MÜller, NÄgeli and Askenasy, von KÖlliker, Mivart, Romanes and others. The differentiation and elaboration of Darwin's theories has gone ever farther and farther; the grades and shades of doctrine held by his disciples are now almost beyond reckoning.