 Other arguments have been mustered against species essentialism. Hull (1965) contends that species have vague boundaries and that such vagueness is incompatible with the existence of species specific essences. According to Hull, essentialist definitions of natural kinds require strict boundaries between those kinds. But the boundaries between species are vague. In all but a few cases, speciation is a long and gradual process such that there is no principled way to draw a precise boundary between one species and the next. As a result, species cannot be given essentialist definitions. (Hull’s argument against species essentialism is very similar to one of Locke’s (1690[1975], III, vi) arguments against kind essentialism.)