All living organisms require food and space. The power of multiplication
in plants and animals is so great that food or space is sooner or later
entrenched upon, and then commences this inevitable struggle for
existence. 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. Pasteur saved the
silk industry of France, and perhaps of the whole world, by the
application of this law of artificial selection. The disease of
silkworms, known as Pebrine, was spreading with ruinous rapidity in
France. Pasteur demonstrated that the germ of the disease could be
detected in the blood of affected moths by the aid of the microscope. He
proved that the eggs of diseased moths produced unhealthy worms, and he
advised that the eggs of each moth be kept apart, until the moth was
examined for germs. If these were found, the eggs were to be burned.
Thus the eggs of unhealthy moths were never hatched, and artificial
selection of healthy stock stamped out a disease, and saved a great
industry.

Each individual plant in the struggle for life has only itself to
maintain. In the higher forms of animal life, each animal has its