 If nature made species or genera by giving all members of the same kind the same form or mould (or, he seems to suggest above, real essence), then there would be no way to explain generative variation in nature (he reiterates this kind of argument in III.vi.15–17) or the differences between samples of the same species of chemicals or minerals. On the other hand, if nature makes real essences, which are material structures that cause the observable qualities of each substance, then these kinds of variations can be explained by variations among the physical structures of the bodies (see Jolley 1999, pp. 144–50).