But it is _the doctrine of descent based upon natural selection_ that is Darwinism itself. The Characteristic Features of Darwinism. 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 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.