tinct, and leave no modified
descendants; and consequently that of the species living at any one
period, extremely few will transmit descendants to a remote futurity. I
shall have to return to this subject in the chapter on Classification,
but I may add that on this view of extremely few of the more ancient
species having transmitted descendants, and on the view of all the
descendants of the same species making a class, we can understand how
it is that there exist but very few classes in each main division of
the animal and vegetable kingdoms. Although extremely few of the most
ancient species may now have living and modified descendants, yet at the
most remote geological period, the earth may have been as well peopled
with many species of many genera, families, orders, and classes, as at
the present day.

SUMMARY OF CHAPTER.

If during the long course of ages and under varying conditions of life,
organic beings vary at all in the several parts of their organisation,
and I think this cannot be disputed; if there be, owing to the high
geometrical powers of increase of each species, at some age, season, or
year, a severe struggle for life, 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 existence,
causing an infinite diversity in structure, constitution, and habits, to
be advantageous to them, I think it would be a most extraordinary fact
if no variation ever had occurred useful to each being's own welfare, in
the same way as so many variations have occurred useful to man. But if
variations useful to any organic being 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 they
will tend to produce offspring similarly characterised. This principle
of preservation, I have called, for the sake of brevity, Natural
Selection. Natural selection, on the principle of qualities being
inherited at corresponding ages, can modify the egg, seed, or young, as
easily as the adult. Amongst many animals, sexual selection will give
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 with
other males.

Whether natural selection has really thus acted in nature, i