War of the Classes by Jack London

Evolution is no longer a mere tentative hypothesis. One by one, step by step, each division and subdivision of science has contributed its evidence, until now the case is complete and the verdict rendered. While there is still discussion as to the method of evolution, none the less, as a process sufficient to explain all biological phenomena, all differentiations of life into widely diverse species, families, and even kingdoms, evolution is flatly accepted. Likewise has been accepted its law of development: _That_, _in the struggle for existence_, _the strong and fit and the progeny of the strong and fit have a better opportunity for survival than the weak and less fit and the progeny of the weak and less fit_.

It is in the struggle of the species with other species and against all other hostile forces in the environment, that this law operates; also in the struggle between the individuals of the same species. In this struggle, which is for food and shelter, the weak individuals must obviously win less food and shelter than the strong. Because of this, their hold on life relaxes and they are eliminated. And for the same reason that they may not win for themselves adequate food and shelter, the weak cannot give to their progeny the chance for survival that the strong give. And thus, since the weak are prone to beget weakness, the species is constantly purged of its inefficient members.