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