This is how it comes about that barriers of language and of culture, no less than barriers of descent, are powerless in face of a well-defined γένος with a potent φύσις of its own. Such a γἑνος can add to the number of individuals which compose it. Pelasgians and Lelegians can _become_ Hellenes. For Herodotus, as I have explained more in detail elsewhere, the process of conversion of barbarians to the Hellenic φύσις is not clear: the verbs which he employs, μετέβαλον, μετέμαθον, are intransitive; the general impression which is conveyed is of a kind of spontaneous generation: and the same language is used when τὸ Ἐλληνικόν is described as ἀποσχισθὲν ἀπὸ τοῦ βαρβἀρου, in the earliest phase of all. For Thucydides, on the other hand--as was natural to an Athenian who had seen Atticism triumphant in Hellas--Hellenism is acquired by contact with, and imitation of, the φύσις of a genuine Hellene. Of course this explanation of Pelasgian conversion only pushes the problem itself one stage further back; but it marks a distinct advance in analysis beyond the point reached by Herodotus; and it is an advance in precisely the opposite direction to that in which naturalists like Hippocrates were being led through their greater insistence on the external factors, which were the main subject of their study. Thucydides in fact stands already on the Socratic side of the line. The explanation of the transmissibility of culture is to be sought for him not in physiology, but in psychology--not in spontaneous or coercive adjustment to inexorable nature, but in intercourse with enlightened minds. Among the many different classes of information which Herodotus inclines to give about foreign peoples, two kinds of data are more insistently recorded than the others. There are the marriage customs, and the principal source of food. These will be admitted to be obvious points to note; but there was a special motive in the fifth century for collecting each of them; and the history of thought in the century which followed allows us to trace this motive forward into a maturer context.