What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include the same bones, in the same relative positions?
This passage, from Chapter XIII, argues that similar bone and body structures in different species illustrate how species may have evolved from a common parent species. The hands of men, the paws of moles, the legs of horses, the paddles of porpoises, and the wings of bats all have the same placement in the body, as well as the same bone structure. However, these structures have evolved differently, creating hands, paws, paddles, and wings. And the divergence of each is particularly well suited to the species to which it belongs: Humans can grasp, porpoises can swim, and bats can fly. In this quotation, Darwin suggests that all of these structures may have evolved from the same structure of the original species, and that natural selection perpetuated the variations that have turned that original structure into the structures of today. Darwin’s mention of human hands is controversial, as it explicitly suggests that humans are subject to descent with modification.