This odious inheritance was the habit of incest between father and daughter, which we have found to be common to all the mammalia as a dominant domestic feature. As a factor in evolution we have seen that it actually had as direct outcome the primal law itself, and thus, with a strange irony, it may be said to have so laid the foundation of an ultimate moral sense. In such or other action in the past it had, however, served its useful purpose. Its operation in the future could be but detrimental; so opposed to all advance does it become, that, as we shall find, it is to be finally swept aside so completely as to permit to some students doubts of its existence, though ‘In Saturn’s time such mixture was not held a crime.’ Leaving no traces of action in actual usage save in such exceptions as prove the rule, it will not be difficult to show that, in giving birth to the primal law, it doomed its own existence, and this apart from any ethical connection. The continued progress of society led almost mechanically to developments eminently inimical to its continuance as a custom, whilst again it would be found even injurious to the order of things as constituted in the earliest group-_plus_-tribe stage. If we bear in mind the axiom that, other things being equal, the largest assemblage of individuals in amity would have the greatest chance of survival, as possessing more numerous units of strength, then father-and-daughter marriage would be pernicious by preventing an assembly from profiting to a full extent by the productive powers of all its members. For such incest implies sole marital rights by a senior generation of males over a junior generation of females. As the latter would always, from mere disparity of age, outlive the former, it follows that, on the death of a father the daughters would remain unproductive, the only other males in the family group being their own brothers, and as such barred to them by the primal law. This situation, in itself an element of weakness, became doubly so, if, as is probable, these young widowed females seceded to other and hostile groups with whom union to them was free. Such groups would in consequence be by so much strengthened, at the expense of their original circle. If, on the other hand, these widows remained in their own circle, their presence as useless mouths would be embarrassing, and a possible source of danger as a temptation to outside suitors. Again, celibacy being quite an anomaly in such an era, complications might arise from possible infractions of the primal law itself within a group.