One of the arguments Darwin uses to convince his readers of natural selection is the similarities of species in separate parts of the world, such as plants and animals of American islands being similar to those of the American mainland. According to Darwin, this “implies, on the theory of descent with modification, that the same parents formerly inhabited both areas” (Darwin, p. 499). Other examples of shared ancestors are the same framework of bones existing within radically different creatures—from the wing of a bat to the leg of the horse, and the similarities of embryos across species, both of which show evidence of slow and slight modifications (Darwin, p. 499). Darwin also discusses disuse and rudimentary organs, where animals in the early stages of life have inherited traits which will never become fully developed, such as the teeth of a calf. Darwin’s evolutionary theory would tell us that such traits have been passed on from a remote period to present day, continuing to reside but remaining undeveloped due to their lack of purpose. (Darwin, p. 500).
I think the most convincing of Darwin’s arguments is that of the shared connections between the species—the fact that similar bone structures exist between various lifeforms seems to indicate that there is a shared ancestor between them, and that there are so many would show this is not just a few species that fall under Darwin’s theory, but the entirety of the existent species on Earth. I would consider his weakest arguments the ones he used to address critiques of his theory—such as the question of geological proof, to which his response essentially boils down to the fact that he thought geology was less reliable than his own conjectures (Darwin, p. 486).