CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.
11. 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 in structure, habits and constitution, by so much
the more can a large number be supported on the area, of which we see proof
by looking to the inhabitants of any small spot, and to the productions
naturalised in foreign lands. Therefore, during the modification of the
descendants of any one species, and during the incessant struggle of all
species to increase in numbers, the more diversified the descendants
become, the better will be their chance of success in the battle for life.
Thus the small differences distinguishing varieties of the same species,
steadily tend to increase, till they equal the greater differences between
species of the same genus, or even of distinct genera.