When we pass from the United States to Mexico we cross the boundary line which separates the two most immense experiments in human breeding the world has ever seen. North of this experimental Rubicon, as we have just seen, the basal stock, which is north-west European or Nordic in origin, has been ruled by a sense of race-caste and has consequently maintained its racial characters. But south of our Rubicon the result of racial contact has been absolutely different. The south-west European or Iberian stock broke down the natural barrier which Nature had set up between them and the natives of Mexico and South America and solved their racial antagonisms by the fusion of blood. The results of these two experiments, carried out on such an immense scale, we can see to-day. The northern experiment, which is now three centuries old, has given the world two of her most virile peoples destined to hold their place whether humanity becomes planted out on a vast, peaceful, and uniform cabbage-patch or still remains, as now, broken up into national and racial factions. These northern peoples are as effective, so far at least as concerns their chances of survival, as the original Nordic stock. The southern experiment, which began four centuries ago, has given the world a jangling series of small peoples, not any one of which is equal, either in body or in mind, to the pioneer Iberian stock. From the anthropologist's point of view the northern experiment is the successful one. We have been glancing at the national and racial problems of the American continent and we ought now to pass on to note the form in which they are presented to us by Australasia. Before passing on, however, there is one very important aspect of the southern or Iberian experiment which we must consider now because it throws light on the path along which I want to lead you. Why did the racial barrier between Iberian and Indian break down? Was it because the Iberian did not possess--was not influenced by--a sense of race-caste such as we have seen to dominate the Nordic colonist? I believe that race-caste or race-prejudice is and has been a more potent force in the Nordic than in the Iberian stock. The Iberian people are near neighbours of the African races; in physique they are differentiated from the African stocks in a somewhat less degree than the Nordic stocks which represent the utmost point in the physical specialization of all European peoples. If the Iberian pioneer carried with him to America a lesser degree of race-caste, the process of hybridization would begin the more easily. The great north and south experiments differed, however, in another important circumstance. The Nordic encountered a scattered, nomadic, proud race; the Iberian a settled people living in dense communities. The Iberian was thus exposed to conditions in which a racial barrier was harder to maintain. STRUGGLE BETWEEN RACE AND SEX IMPULSES