The second group of causes which is most potent in developing distinct races of domesticated animals—namely, conscious selection—has probably never been very active in the races of man. We do not know of a single case in which it can be shown that intermarriage between distinct types of the same descent was prohibited; and whatever selection there may have been in the development of primitive society seems to have been rather that type of natural selection which encourages the mating of like with like, or such intricate selection as is due to the social laws of intermarriage, which prevented intermarriages of relatives of certain grades, and often also of members of different generations. Thus a very common form of marriage restriction brings it about that among certain tribes the children of brother and sister intermarry, while the children of brothers and the children of sisters are not allowed to intermarry. Similar restrictions are found in great number, and may possibly have had a certain selective effect, although their operation can hardly be assumed to have had very marked results upon the form of the human body (Pearson). In some cases social laws have had the indirect effect of perpetuating distinctions between separate parts of a population, or at least of retarding their complete amalgamation. This is the case where laws of endogamy relate to groups of distinct descent, and may be observed, for instance, among the castes of Bengal, where the low castes are of the characteristic South Indian type, while the highest castes preserve the type of the tribes of northwestern India (Risley and Gait). The numerous intermediate castes show, however, that the laws of endogamy, even where they are as stringent as those of India, cannot prevent blood-mixture. Whether or not in extreme cases endogamy in small groups, as among the ancient Egyptians, has led to the development of well-defined types, is a question that cannot be answered; but it is certain that none of these types, when found in a large population, have survived.