Whether man inherited the feeling from the predecessors from whom he sprang, or whether it was developed after the evolution of distinctly human qualities, we do not know. It must necessarily have arisen at a stage when family ties became comparatively strong, and children remained with their parents until the age of puberty, or even longer. Exogamy, as a natural extension of this instinct, would arise when single families united in small hordes. It could not but grow up if the idea of union between persons intimately associated with one another was an object of innate repugnance. There is no real reason why we should assume, as so many anthropologists have done,[2054] that primitive men lived in small endogamous communities, practising incest in every degree. The theory does not accord with what is known of the customs of existing savages; and it accounts for no facts which may not be otherwise far more satisfactorily explained. The objection will perhaps be made that the aversion to sexual intercourse between persons living very closely together from early youth is too complicated a mental phenomenon to be a true instinct, acquired through spontaneous variations intensified by natural selection. But there are instincts just as complicated as this feeling, which, in fact, only implies that disgust is associated with the idea of sexual intercourse between persons who have lived in a long-continued, intimate relationship from a period of life at which the action of desire is naturally out of the question. This association is no matter of course, and certainly cannot be explained by the mere liking for novelty. It has all the characteristics of a real, powerful instinct, and bears evidently a close resemblance to the aversion to sexual intercourse with individuals belonging to another species. * * * * *