Darwin’s final arguments, meant to summarize his view and convince his readers, began with a few statements that he thinks everyone would agree with. The first being that nature has variation. Species of birds can have variations without being an entirely kind, as well as the reverse. He also expected it to be excepted that all species, no matter where or what, are always in a fight for survival, no matter how minor, and that, as a result, different species are in competition for resources since they will have different strengths and weaknesses. In his argument for natural selection, he wasn’t saying that one day, all parrots woke up with curved beaks, but that rather, the parrots that did have them (through an anomaly in breeding, for example) were better able to survive and had more offspring than their competitors. Eventually, as generations passed, the differences between groups would become so great that they could no longer interbreed, and then they should be considered different species.
I don’t need much convincing personally, but the part of the argument that strikes me as the most most interesting and in that way, the most convincing, is when he referenced how animals that have been thrown into an environment they aren’t adapted too or aren’t suited for can lose their fertility- such as certain species held in captivity, even if they are being held not far from where they used to live and in superficially similar conditions. There is a lot of factors that we don’t know or understand, but if something like that can hinder the production of offspring, it isn’t hard to imagine that small changes over time (to the environment) can change how a species expresses traits so that it can continue to survive and produce offspring.