That this profound change in the mental qualities of the population of Greece was due to substitution of one racial stock by an inferior one is beyond question. That a great change of racial type was effected is sufficiently proved by the comparison of the physical type of the modern with that of the ancient Greeks. The modern are predominantly dark and round headed; the ancient were distinctly long headed, as shown by a sufficient number of skull measurements; and they were, as regards the dominant class at least, predominantly fair in colour. It has been supposed that the many references to the fair hair and complexion of heroes and gods were due to fair persons being very rare and hence an object of special admiration; but there is no ground for this. The way in which this racial substitution took place is also pretty clear; and the rapid, almost sudden, decline of the intellectual productivity of the Greek people coincided in time with the racial change. The first and most important factor in the extermination of the best blood of ancient Greece was military selection. Military group selection in the prehistoric period had, no doubt, played a great part in bringing about the evolution of the superior mental qualities of the European peoples, especially of the fair northern race. So long as the peoples consisted of more or less wandering tribes of pure race, which waged a war of extermination upon one another, the peoples and tribes of superior mental and moral endowments must in the main have survived, while those of inferior endowments went under. But, so soon as the Nordic tribes became settled as aristocracies ruling over the Pelasgian populations, the effects of military selection tended to be reversed; instead of making for racial improvement, they made for deterioration. That racial deterioration occurs under these conditions seems to be an almost general law; it has been exemplified among many different peoples. The many small Greek states were almost perpetually at war with one another; and the result of the warfare was not so much the wholesale extermination of the people of any one state, as the killing off in large numbers of the younger men of the ruling caste, the free citizens of whom the armies were almost entirely composed. The wars between Sparta and Athens were the most destructive and tragic of all in this respect. We know that the numbers of Spartans of the aristocratic class, never very large, became fewer and fewer, in spite of efforts made to keep up the number by admitting to citizenship persons not of pure Spartan blood; and that Sparta was eventually destroyed simply for lack of men, men of the ruling class[122].