A theory which seeks to explain by natural causes the occurrence of every kind of adaptation which is to be met with in organic nature, on the assumption that adaptations of every kind have primary reference to the preservation of species, and therefore, also, as a general rule, to the preservation of their constituent individuals.
This theory was first proposed by Darwin and is based on the following principles: overproduction of organisms and the consequent struggle for existence; the variations of organisms, useful and harmful, and the preservation of the useful, and destruction of the harmful; overcrowding; accidental destruction; change of habits; and migration.
This theory is also termed the survival of the fittest (Herbert Spencer).
It has not been established by fact, and is not now advanced even as a hypothesis to explain the origin of species.
New Catholic Dictionary