ut reproducing, the idea came to him that the decision as
to which member of a species was to perish and which was to attain to
maturity and reproduction might not be a matter of chance, but might
be determined by the constitution of the individuals themselves,
according as they were more or less fitted for survival. With this
idea the foundation of the theory of selection was laid.

In _artificial selection_ the breeder chooses out for pairing only
such individuals as possess the character desired by him in a somewhat
higher degree than the rest of the race. Some of the descendants
inherit this character, often in a still higher degree, and if this
method be pursued throughout several generations, the race is
transformed in respect of that particular character.

_Natural selection_ depends on the same three factors as _artificial
selection_: on _variability_, _inheritance_, and _selection for
breeding_, but this last is here carried out not by a breeder but by
what Darwin called the "struggle for existence." This last factor is
one of the special features of the Darwinian conception of nature.
That there are carnivorous animals which take heavy toll in every
generation of the progeny of the animals on which they prey, and that
there are herbivores which decimate the plants in every generation had
long been known, but it is only since Darwin's time that sufficient
attention has been paid to the facts that, in addition to this regular
destruction, there exists between the members of a species a keen
competition for space and food, which limits multiplication, and that
numerous individuals of each species perish because of unfavourable
climatic conditions. The "struggle for existence," which Darwin
regarded as taking the place of the human breeder in free nature, is
not a direct struggle between carnivores and their prey, but is the
assumed competition for survival between individuals _of the same_
species, of which, on an average, only those survive to reproduce
which have the greatest power of resistance, while the others, less
favourably constituted, perish early. This struggle is so keen, that,
within a limited area, where the conditions of life have long remained
unchanged, of every species, whatever be the degree of fertility, only
two, _on an average_, of the descendants of each pair survive; the
others succumb either to enemies, or to disadvantages of climate, or
to accident. A high degree of fertility is thus no