 Kitcher is correct that biologists attempt to explain the traits of organisms in two ways: sometimes they cite the ultimate, or evolutionary, cause of a trait; other times they cite a structural feature of an organism with that trait. A problem with Kitcher’s approach is his characterization of biological practice. Biologists since Darwin have taken species to be evolutionary units. A glance at a biology text book will reveal that the evolutionary approach to species is the going concern in biology. The groups that correspond to Kitcher’s structural concepts are not considered species by taxonomists. Groups of organisms that have genetic, developmental, behavioral and ecological similarities, are natural kinds in biology, but they are not considered species. Consider such groups of organisms as males, females, tree nesters and diploid organisms. These groups of organisms cut across species. For instance, some but not all humans are males and many organisms in other species are males. Male is a kind in biology, but it is not a species. Kitcher’s motivation for asserting that species are sets is to allow spatiotemporally unrestricted groups of organisms to form species. That motivation, however, is not substantiated by biological theory or practice.