Darwin argued that his theory of natural selection and by extension, evolution, was correct through several points. The first point summarized is the matter of hybrid species. Darwin points out that some hybrids are fertile, while other others are not. We humans do not bat an eye at this, but Darwin reasons that animal species with similar forms can produce increasingly fertile offspring. (Darwin, 484.) He moves onto to something he observed in the Galapagos, the impact of geology on the evolution of species. He reasons that the climate of an area will kill off a species until a member adapts and breeds. A better adapted variety would spring from these intermediate varieties, and kill the intermediates off. (Darwin, 486.)
I think the geological argument is the most convincing. Darwin is known to have taken and preserved bird species from the Galapagos islands. This gives visual aids to his theories of natural selection and evolution. It is also an easier to understand explanation, as only animal breeders and some scientists would be able to understand Darwin’s ideas on animal hybrids. If we take an average Joe from the street, and present to him both explanations, he would choose the one he could see better, not the one that would require a few hours of background information. For example, this chapter was somewhat difficult to decipher information from, but the easiest point to understand was the one about geological variation affecting how species developed. Thus, the second explanation is the best in my view.