Versus the study of Variation among individuals per se. Owing to this struggle, variations, however, slight and from whatever cause proceeding, if they be in any degree profitable to the individuals of a species, in their infinitely complex relations to other organic beings and to their physical conditions of life, will tend to the preservation of such individuals, and will generally be inherited by the offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of a species which are periodically born, but a small number survive. What this means is that the concept of natural selection results as an inevitable outcome of the acceptance of certain biological facts. Fact 1 .
Natural populations have enormous excess fecundity.
Fact 2 . Population sizes remain stable. Fact 3 . Limitation of resources. A severe struggle for existence must occur.