In this struggle for life, the individuals best able to conform to their environment, _i.e._, the best able to resist adverse circumstances, to sustain hardships, to overcome difficulties, to defend themselves, to outstrip their fellows, in short, to harmonise function with environment, survive.

These propagate their kind according to the law of heredity.

Variations exist in the progeny, and the individuals whose variations best adapt them to their environment are the fittest to, and do, survive.
In a state of nature the weaklings perish.

If man interferes with this state of nature in the lower animals, he may make a selection and cultivate some particular attribute.

This is artificial selection, and is best exemplified in the experiments with pigeons.