augmentation of specific forms.

SUMMARY OF CHAPTER.

If under changing conditions of life organic beings present individual
differences in almost every part of their structure, and this cannot be
disputed; if there be, owing to their geometrical rate of increase, a
severe struggle for life at some age, season or year, and this certainly
cannot be disputed; then, considering the infinite complexity of the
relations of all organic beings to each other and to their conditions
of life, causing an infinite diversity in structure, constitution, and
habits, to be advantageous to them, it would be a most extraordinary
fact if no variations had ever occurred useful to each being's own
welfare, in the same manner as so many variations have occurred useful
to man. But if variations useful to any organic being ever do occur,
assuredly individuals thus characterised will have the best chance of
being preserved in the struggle for life; and from the strong principle
of inheritance, these will tend to produce offspring similarly
characterised. This principle of preservation, or the survival of the
fittest, I have called natural selection. It leads to the improvement
of each creature in relation to its organic and inorganic conditions of
life; and consequently, in most cases, to what must be regarded as an
advance in organisation. Nevertheless, low and simple forms will long
endure if well fitted for their simple conditions of life.

Natural selection, on the principle of qualities being inherited at
corresponding ages, can modify the egg, seed, or young as easily as the
adult. Among many animals sexual selection will have given its aid to
ordinary selection by assuring to the most vigorous and best adapted
males the greatest number of offspring. Sexual selection will also give
characters useful to the males alone in their struggles or rivalry with
other males; and these characters will be transmitted to one sex or to
both sexes, according to the form of inheritance which prevails.

Whether natural selection has really thus acted in adapting the various
forms of life to their several conditions and stations, must be judged
by the general tenour and balance of evidence given in the following
chapters. But we have already seen how it entails extinction; and how
largely extinction has acted in the world's history, geology plainly
declares. Natural selection, also, leads to divergence of character; for
the more organic beings diverge i