I have entered upon this long discussion because I believe that we require to appreciate this power of persistence on the part of the sexual determinants before we can explain the general occurrence of amphigony. The occurrence of pure parthenogenesis, unaccompanied by any degeneration of the species, can hardly be understood except on the assumption that the constancy of the species, when it has once been attained, may be preserved without the continual intervention of amphimixis. How long it can be preserved is another question, which it is difficult or impossible to answer, since species exhibiting _pure_ parthenogenesis are rare, and since we cannot tell with certainty how long it is since amphimixis ceased to occur in them. Generally speaking, the answer in regard to the few species which have to be taken into account in this connexion would be 'not long,' but whether this 'not long' signifies hundreds of generations or thousands of generations we must leave undecided. So much only we can say, that in all species of animals in which the male sex has quite died out or has dwindled to a minimal remnant, there are as yet no traces of degeneration to be found, and that even organs which have fallen into disuse and become functionless because amphigony has disappeared, are nevertheless in several cases retained in perfect completeness. I shall return to this subject later on, but in the meantime I wish to work out our conception of the actual efficacy of amphigony or ordinary sexual reproduction, and thereby increase our understanding of its significance and power of persistence. We have seen that amphigony not only renders possible the novel 'harmonious adaptations' which are continually required, but that it also leads, by a continual crossing of individuals, simultaneously with the elimination of the less fit, to a gradually increasing constancy of the species. This has been regarded by some writers as its sole effect; thus recently by Hatschek, whose view has already been refuted. Haycraft also finds the significance of amphigony simply in the equalizing or neutralizing of individual differences which it effects. Quetelet and Galton have attempted to show that intercrossing leads to a mean which then remains constant. Haycraft supposes that a species can only remain constant if its individuals are being continually intercrossed, and that otherwise they would diverge and take different forms, because the 'protoplasm' has within itself the tendency to continual variation. The transformation of species is effected by means of this variation tendency, and the persistency and constancy of species which are already adapted to the conditions of their life are secured by the constant intercrossing of the individuals, and the consequent neutralization of individual peculiarities.