Aristotle on Species Variation

This article will consider whether Aristotle admitted or denied the possibility of continuous variation in the category of substance, that is, the possibility that there should be two species with an infinite gradation of species in between. (Here 'species' is used in the Aristotelian sense of 'natural kind', and so is not confined to biological species.) It will be concluded that he asserted that continuous variation between species was not only possible but in fact occurred in an important case. The question is interesting for two reasons. Firstly, the scholastic version of Aristotelianism received one of its most telling criticisms on precisely this point. Locke attacked the scholastics' realism about universals on various a priori grounds, but the principal observable phenomena which he adduced as incompatible with their theory were examples of continuous variation between (supposed) species. He argued, for example, that intermediate forms such as fishes with wings, amphibious animals, mermaids and so on show that there are no real boundaries between species, and concluded that things are sorted by us only according to our own ideas of them.1 Hence, he thought, there were no real species. In the case of the human species, in particular, he believed that the existence of 'like-men' with language and reason, but with hairy tails, showed that the boundary of the species is arbitrary, so that what we decide to call men depends not on any real essence but 'only our abstract ideas'.2 But these considerations would not count