 While noting differences in skin tone, he based his varieties upon the structures of the cranium, which supposedly gave his distinctions a stronger scientific foundation than the more superficial characteristic of color (Hannaford 1996, 206). In addition, he strongly denied polygenetic accounts of racial difference, noting the ability of members of different varieties to breed with each other, something that humans were incapable of doing with other species. Indeed, he took great pains to dismiss as spurious accounts of Africans mating with apes or of monstrous creatures formed through the union of humans with other animals (Hannaford 1996, 208–209). In final support of his more scientific, monogenist approach, Blumenbach posited the internal, biological force which generated racial difference, the “nisus formativus,” which when triggered by specific environmental stimuli generated the variations found within the varieties of humans (Hannaford 1996, 212).