Figure 1.

Darwin's graphical representation of the principle of descent with modification and how new varieties would be formed over long periods of time by natural selection. The intervals indicated by Roman numerals "may represent each a thousand or more generations" (Origin of Species, 6th edition, p85). The capital letters along the bottom indicate original species. "The little fan of diverging dotted lines of unequal lengths proceeding from (A), may represent its varying offspring. The variations are supposed to be extremely slight, but of the most diversified nature; they are not supposed all to appear simultaneously, but often after long intervals of time; nor are they all supposed to endure for equal periods. Only those variations which are in some way profitable will be preserved or naturally selected." So, after a thousand generations, "species (A) is supposed to have produced two fairly well-marked varieties, namely a¹ and m¹", and so on. Darwin explains that he has chosen the "extreme species (A) and the nearly extreme species (I) as those which have largely varied, and have given rise to new varieties and species".