We observe from the earliest times a rhythmic play, as it were, of opposite forces that tends, alternately, to build up and to break down and mingle human races, but of the laws that underlie and govern these forces we know little or nothing. On the one hand we see how man has always and everywhere shown what the advocates of so-called racial purity have called "a perverse predisposition to mismate" which has made it exceedingly difficult to classify existing human varieties. On the other hand we see throughout nature how a pronounced disparity between varieties of the same species engenders an aversion from one another of the different varieties which seems to arise, in men and animals alike, through the instinct of sexual jealousy which is probably bound up with the primary instinct of self-preservation. Those people who profess belief in the inherent superiority of a particular race naturally look upon the tendency towards race-blending as a perverse proclivity, while those who think that all men are potentially equal regard it as a wholesome instinct provided by nature to counteract the feebleness and infertility which cause the dying-out of the race that becomes too pure. Racial antipathy seems to depend in the degree of its strength upon the degree of physical disparity between given races. In the so-called Latin races of to-day, prejudice against black people is certainly weaker than in the blond races of Northern Europe. Is this aversion a matter of absolute instinct or is it an acquired social characteristic and as such liable to change? I think the answer must be that this racial repugnance is not naturally inherent in children, nor in women towards the men of a different kind, nor in men towards the women of another race, but that it arises naturally and spontaneously and, in this sense, instinctively, through the feeling of jealousy which is caused, in both men and women, by fear of losing their natural mates to rivals of both sexes from another and disparate race.