It is the more important to keep in mind this fundamental conception of Greek physical anthropology when we go on to consider either the treatment of the evidence of language and culture, which we find in Herodotus, or the applications of physical classification to the purposes of logic and metaphysic. To take the latter first: a doctrine of the real existence of natural kinds, corresponding each, as Hippocrates would put it, to a process of growth peculiar to itself, was clearly easier to understand, if not to discover and formulate, when the men who were to discuss it were already brought up to regard the animal world, for example, as consisting of a comparatively small number of fundamental types, and the infinite variety of individual and regional forms as the effect of external forces upon them. Each actual example of horse or dog, for example, was to be regarded on the one hand as the embodiment of a true equine or canine nature, which reason might hope to detect and isolate; but on the other, it lay like the god Glaucus, encrusted with accidental qualities, the effects of its exposure to a particular environment. Seen in the light of their pre-Socratic history, as elements in the terminology of a great school of naturalists, the catch-words φύσις, γένος, εἶδος, and συμβεβηκὸς gain something, I think, in significance. In particular, it becomes clearer why the word εἶδος, which continued to be used among the naturalists for the specific outcome of συμβεβηκότα upon a member or members of a γένος, came among the philosophers to supersede the word γἑνος in proportion as the centre of reflective interest shifted from the objective exponent of a φύσις to the subjective standpoint of the philosophic observer.