different
organs (_e.g._, cats with white fur and blue eyes are also deaf).

This is _natural selection by means of the survival of the fittest in the
struggle for existence_. Changes in the conditions and surroundings of
life, and more or less {41} perfect adaptation of the organisms to the new
conditions of form, color, food and habit, are the main causes of those
individual variations, the accumulation of which through many generations
produces so great effects. If we only have behind us periods long enough to
permit us to imagine each step in the development as an extremely small and
hardly appreciable one, natural selection offers us not the exclusive but
the main means of explaining the evolution of the whole animal and plant
world out of one or a few simple organized original forms.

This is the outline of the selection theory, as given by Darwin in 1859,
and still retained in all its essentials. It is true, in his work on the
origin of man he added as supplemental the _sexual_ to the common natural
selection, and made it of special importance for the presentation of the
_beautiful_ in nature--for the production of beautiful forms, colors, and
tones, and for the development of power and intelligence. And in the same
work he said that there are many circumstances of structure which seem to
be neither beneficial nor detrimental to the individual, and that to have
overlooked this fact was one of his greatest mistakes in his former
publications. But for the rest, he maintains the selection theory
unchanged, with the single modification that it explains, if not the whole
development of the species through descent, at least that which is of most
importance in it.

That it was only one step in the course of reasoning to extend the
selection theory to the descent of man, was seen by many as soon as
Darwin's work on the origin of species was published and began to attract
{42} attention; although not a syllable upon this question was presented in
this work. Various persons manifested their presentiment or perception
according to their point of view--partly by the most violent opposition to
the new doctrine, partly by scientific development or modification of their
anthropogonic views, partly also by revelling in imagination in the
consequences hostile to religious faith which they thought could be drawn
from this doctrine. We remind the reader of the itinerant lectures of Karl
Vogt about the ape-pedigree of man, and of