A third way in which the fusion by conquest of two peoples must have made for progress was by biological blending, the crossing by intermarriage of the two stocks. We have seen that there is a considerable amount of evidence to show that, when two stocks are very widely different in mental and physical characters, the result of crossing is likely to be bad, the crossed race is likely to be inferior to, and less fit for the battle of life than, both parental stocks; the characters of individuals will be apt to be made up of a number of elements more or less inconsistent with one another; such a composite character made up of inharmonious elements will be apt to be unstable and constantly at war with itself. Character of this kind and the tragic struggles to which it is liable to find itself committed has been well described in fiction by a number of authors, especially in stories of the Mulattoes of America. On the physical side it has been shown that such cross-bred races tend to die out owing to lack of balance of the physical constitution. On the other hand, we saw that the crossing of two closely allied racial stocks seems to have a tendency to produce a cross-bred race superior to both parent stocks, and especially to produce a variable stock. It is, I think, probable that the frequently repeated blending of allied stocks in Europe has been the fundamental biological condition of the capacity of the European peoples for progressive national life. In the case of the conquest of one people by another differing very markedly in racial qualities, there seem to be two alternatives equally prejudicial to the continued progress of the nation so formed. On the one hand, free intermarriage may take place, resulting in an inferior cross-bred race incapable of high civilisation, as seems to have occurred in most of the countries of South America, where it is with the greatest difficulty that the outward forms of the high civilisation which they have imitated from Europe are maintained. On the other hand, where especially the outward physical characters are very different, the conquering people may hold itself apart from the conquered, and maintain itself as a ruling class, which prides itself on the purity of its blood and which tends to harden into a caste. Such conquest without subsequent blending gives rise to a civilisation which, being founded upon a rigid caste system, is incapable of continued progress. This is what has happened in India. The fair-skinned Aryan invaders despised the dark-skinned indigenous peoples, whom they spoke of as being scarcely human, and, in spite of a good deal of crossing, they have in theory and in the case of the Brahmans at least to a considerable extent in practice, maintained the purity of their blood, by means of the development of the caste system.