He then explains that an incipient species that happens to develop can have one of three fates: 1) they may become instinct, 2) they may remain as a variety for a very long time, or 3) the variety can meet success and develop into a distinct species. Darwin attributes "the passage of a variety, from a state in which it differs very slightly from its parent to one in which it differs more, to the action of natural selection" (Darwin 52). This is the crucial link between the two main points of Darwin's argument: that natural selection is the development of different species.
Adding to Darwin's observation that domestic species with a large number of individuals will develop new variations at a higher rate, he then generalizes it to the natural world. The species that flourish "and are the most numerous in individuals, Æ'{which oftenest produce well-marked varieties, or, as I consider them, incipient species" (Darwin 54). This process in theory should propagate itself; if the species in a genus have recently created species, the cause of the original variation should still be in effect and in the future should cause the formation of even more incipient species. When this model is applied to nature, it is supported with the observation that the species of larger, expanding generas have are less defined then smaller generas. The species of the larger genera will be closer on the gradient to incipient species. Thus nature holds consistent with Darwin's account of incipient species.
Darwin's next step in the formation of his argument is to develop natural selection (so far only observed in domestic settings) to nature. Since every single organism produces a large multiple of offspring, in order for populations to remain constant, only a small fraction of individuals born can survive. This causes "a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life" (Darwin 63).
