n the mutual
relations of all organic beings; a conviction as necessary, as it is
difficult to acquire. All that we can do is to keep steadily in mind
that each organic being is striving to increase in a geometrical ratio;
that each, at some period of its life, during some season of the year,
during each generation, or at intervals, has to struggle for life and
to suffer great destruction. When we reflect on this struggle we may
console ourselves with the full belief that the war of nature is not
incessant, that no fear is felt, that death is generally prompt, and
that the vigorous, the healthy, and the happy survive and multiply.

CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.

Natural Selection--its power compared with man's selection--its power
on characters of trifling importance--its power at all ages and on
both sexes--Sexual Selection--On the generality of intercrosses
between individuals of the same species--Circumstances favourable and
unfavourable to the results of Natural Selection, namely, intercrossing,
isolation, number of individuals--Slow action--Extinction caused by
Natural Selection--Divergence of Character, related to the diversity of
inhabitants of any small area and to naturalisation--Action of Natural
Selection, through Divergence of Character and Extinction, on the
descendants from a common parent--Explains the Grouping of all organic
beings--Advance in organisation--Low forms preserved--Convergence of
character--Indefinite multiplication of species--Summary.

How will the struggle for existence, briefly discussed in the last
chapter, act in regard to variation? Can the principle of selection,
which we have seen is so potent in the hands of man, apply under nature?
I think we shall see that it can act most efficiently. Let the endless
number of slight variations and individual differences occurring in our
domestic productions, and, in a lesser degree, in those under nature, be
borne in mind; as well as the strength of the hereditary tendency. Under
domestication, it may truly be said that the whole organisation becomes
in some degree plastic. But the variability, which we almost universally
meet with in our domestic productions is not directly produced, as
Hooker and Asa Gray have well remarked, by man; he can neither originate
varieties nor prevent their occurrence; he can only preserve and
accumulate such as do occur. Unintentionally he exposes organic beings