Context: In On the Origin of Species Darwin puts forth the theory of evolution and says that evolution is caused by natural selection. Nature cannot "take a sudden leap from structure to structure . . . for natural selection acts only by taking advantage of slight successive variations. . . ." In chapter 3 Darwin considers the question of "how species arise in nature. How have all those exquisite adaptations of one part of the organization to another part, and to the conditions of life, and of one organic being to another being, been perfected? . . . we see beautiful adaptations everywhere and in every part of the organic world." How do varieties, Darwin asks, "become ultimately converted into good and distinct species . . .?" The formation of species and of genera results from "the struggle for life":
. . . 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 any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection, in order to mark its relation to man's power of selection . . .