726 THE AMERICAN JOURNAL OF SOCIOLOGY
races may be accounted for as the normal effects of social differ- entiation.
SOCIAL INTEGRATION.
Social differentiation forms the first chapter in the history of human society. The second chapter treats of social integration, and this is as far as it is proposed to go in this paper along this line. In a comparatively short time (and we are not at all limited in the matter of time) the number of human races was very great. These were all absolutely distinct sociologically, and many of them were wholly different ethnologically. They were alike only in their biological aspects. They were all human races. The number of races at a certain stage, viz., at the stage of maximum differentiation, was vastly greater than it is now, and greater than it was at any subsequent stage. This reduction in the number of human races was the result of social integra- tion.
A drainage map of any great continent will show that the large streams that converge in their lower courses are made up of numerous smaller streams higher up, and that these are com- posed of great numbers of minor tributaries on the high plateaus. Many of these latter, when belonging to one of the great drain- age systems, have their ultimate sources close to those belong- ing to the next adjacent system. Let us imagine two of those moniliform chains of human migration that I have described creeping slowly up two of these great drainage basins, the con- strictions between the numerous bead-like links having in most cases so far deepened that all social and racial connection has been repeatedly severed. In both these lines the more remote groups, after reaching the highlands, have further ramified in partial correspondence with the topography, until groups are found in both far up among the ultimate sources of the smaller tributaries. In a tropical or subtropical region, such as that in which the human race could alone have had its origin, these high plateaus must be covered with dense forests. In the pursuit of game and other means of subsistence the more venturesome of these terminal groups could not fail occasionally to cross over the narrow divide that separated the head waters of the two
