Darwin first presents his argument that the nearly universal sterility of species when crossed and the nearly universal fertility of varieties of one species when crossed suggests that any new species is not a result of interbreeding between species, but in fact is a result of breeding within one species that leads to new characteristics through genetic variation (Darwin, 484). He next presents the reader with the argument that we often cannot see a clear gradation of one species transforming into the next because of the imperfection of the geologic record; he states that the temporal and spatial scale of the geologic record is so vast that humans could never hope to find every variation in this gradation, and that when we do find a step in the gradation that we will likely not recognize it as such, instead deeming it a new species (Darwin, 486). Darwin next cites domesticated animals as a case where varieties of a breed are often selected for by man and thereby can “largely influence the character of a breed” (Darwin, 488). He also goes on to argue that individual varieties may seem unimportant but that they add up over time and allow for greater differences between species within a genus (Darwin, 492), and that the wide distribution of many species can be attributed simply to migration, albeit over the course of large spans of time and driven by climate and geographic changes (Darwin, 498). There is, of course, a lot more to his arguments but I’m running out of room for this post.
I found his first argument about the almost universal sterility of two species when crossed extremely compelling. If a new species cannot come into existence through the combining of two separate species, it must come into existence through genetic variations within one species. This seems to be the essence of descent with modification and is an extremely important point to note about Darwin’s writing.