Transformation and differentiation go on in nature as a vast, ceaseless, but blind process of selection. In 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 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. Various Forms of Darwinism.