Evolution by the Numbers. James Wynn

      In a large genus it is probable that more than one species would vary. . . . In each [large] genus, the species, which are already extremely different in character, will generally tend to produce the greatest number of modified descendents; for these will have the best chance of filling new and widely different places in the polity of nature. (99)

      In addition to making the case that bigger is better, Darwin argues by implication that the breadth of diversity of larger genera and species promotes further diversity and population expansion. This means that larger genera and species will most likely continue to grow and outstrip their smaller counterparts, unless checked by some unforeseen circumstances. The supposed correlation between size and variation assumed in this passage suggests that, for Darwin, bigger genera and species are bigger in the first place because they are more diverse. Thus, diversity itself becomes a crucial factor in evolutionary success.