d 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 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 Mueller, Naegeli and
Askenasy, von Koelliker, Mivart, Romanes and others. The differentiation
and elaboration of Darwin's theories has gone ever farther a