The typological theory of species classification, which has few adherents today, distinguishes species solely on the basis of observable physical characteristics, such as plumage color, adult size, or dental structure  However, there are many so-called sibling species,  which are indistinguishable on the basis of their appearance but cannot interbreed and thus, according to the mainstream biological theory of species classification, are separate species
Since the typological theory does not count sibling species as separate species, it is unacceptable
the argument, in its attempt to refute one theory of species classification, presupposes the truth of an opposing theory