Now in view of our argument as regards the origin of these diversities in some sexual feelings, it is a most significant feature in these details of the terms of address that the expression of the relativity of age between the speakers is confined solely to the intercourse between members of the same sex. That a brother is the senior or the junior of Ego is carefully noted, but a sister is simply and vaguely a sister. Why? simply because whereas, by virtue of the primal law, no possible question whatever of mutual interest in sexual matters could possibly arise between a brother and a sister, on the other hand friction might hourly occur between brothers or between sisters. In fact, if our theory is correct, then, as questions of sexual privilege or precedence could cause jealousy between members of the same sex, distinctions would be necessary by definition of seniority when address took place between these, and in these cases alone, and this indeed we find to be the fact. As conclusive evidence we would cite the further important fact that these very same distinctions of senior and junior are used, _inter se_, between all those of the same totem [phratry] as now existing, but are never employed for their tribal cousins of the other totem [phratry]. And the reason is the same. The latter naturally do not marry (in groups formed of only two classes) [phratries] into the same totem [phratry] as the former, and thus there is no cause for jealousy or necessity of definition, whereas individuals of the same totem [phratry] are _ipso facto_ group [potential] husbands of the same group [potential] wives, or are at least eligible in marriage with the same totem groups [phratries], and hence necessity for the exact definition by age of each one’s rights. Thus, as with other laws or institutions we have traced, we find a desire for distinction as regards rights in sexual union to be the genetic cause of the classificatory system both as regards the generation and its component members.