ecies
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 {127} 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 ratio of increase of each species, 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 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
manner 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; and it leads to
the improvement of each creature in relation to its organic and inorganic
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. 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, in modifying and
adapting the various forms of life to their several conditions and
stations, must be judged of by the general tenour and balance of evidence
given in the following chapters.