A study of rabbits in the 1940s convinced many biologists that parthenogenesis -- reproduction without fertilization of an egg -- sometimes occurs in mammals. However, the study' s methods have since been shown to be flawed, and no other studies have succeeded in demonstrating mammalian parthenogenesis. Thus, since parthenogenesis is known to occur in a wide variety of nonmammalian vertebrates, there must be something about mammalian chromosomes that precludes the possibility of parthenogenesis. A flaw in the reasoning of the argument is that the argument ['infers that a characteristic is shared by all nonmammalian vertebrate species merely because it is shared by some nonmammalian vertebrate species', 'confuses a necessary condition for parthenogenesis with a sufficient condition for it', 'takes for granted that something that has not been proven to be true is for that reason shown to be false', "assumes that the methods used in a study of one mammalian species were flawed merely because the study's findings cannot be generalized to all other mammalian species"]