plasticity made it more possible for a new generation to improve upon the ideas and customs of its predecessors.[65] In these two concomitant processes--the development of the family and the increase of mental plasticity, or ability to adopt new methods and strike out into new paths of thought--lies the whole explanation of the moral and intellectual superiority of men over dumb animals. But in each case the change was very gradual.[66] The true savage is only a little less unteachable than the beasts of the field. The savage family is at first barely discernible amid the primitive social chaos in which it had its origin. Along with polyandry and polygyny in various degrees and forms, instances of exclusive pairing, of at least a temporary character, are to be found among the lowest existing savages, and there are reasons for supposing that such may have been the case even in primeval times. But it was impossible for strict monogamy to flourish in the ruder stages of social development; and the kind of family-group that was first clearly and permanently differentiated from the primeval horde was not at all like what civilized people would recognize as a family. It was the _gens_ or _clan_, as we find it exemplified in all stages from the middle period of savagery to the middle period of barbarism. The _gens_ or _clan_ was simply--to define it by a third synonym--the _kin_; it was originally a group of males and females who were traditionally aware of their common descent reckoned in the female line. At this stage of development there was quite generally though not universally prevalent the custom of "exogamy," by which a man was forbidden to marry a woman of his own clan. Among such Australian tribes as have been studied, this primitive restriction upon promiscuity seems to be about the only one. [Footnote 62: [Greek: All' age moi tode eipe kai atrekeos katalexon, ei de ex autoio tosos pais eis Odyseos. ainos gar kephalen te kai ommata kala eoikas keino, epei thama toion emisgometh' alleloisin, prin ge ton es Troien anabemenai, entha per alloi Argeion hoi aristoi eban koiles epi neusin ek tou d' out' Odysea egon idon out' eme keinos. Ten d' au Telemachos pepnymenos antion euda toigar ego toi, xeine, mal' atrekeos agoreuso. meter men t' eme phesi tou emmenai, autar egoge ouk oid'; ou gar po tis heon gonon autos anegno.] _Odyssey_, i. 206.]