What is true of the chick is true of all life. This is the law of evolution. It is true of all plant and animal life; it is true of man as an individual; it is true of his mind as well as of his body; it is true of society as an aggregation of individuals. As men have evolved from a lower to a higher, a simple to a complex state, so they are still evolving and rising "on stepping stones of their dead selves to higher things." Natural selection, or the survival of the fittest, is one of the processes by which evolution takes place. According to this law, only the fittest survive in the struggle for life. Darwin was led to this discovery on reading Malthus's thesis regarding the disproportion between the rates of increase in population and food, and the consequent struggle for existence. 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.