aturity, and that,
therefore, the greater number of the descendants of a species perish
without 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 d