towards their own improvement. But for this amphimixis is essential; consequently the descendants which have arisen through amphimixis, and whose ancestors have arisen in the same way, have an advantage over those of parthenogenetic origin. On the whole, at least, this must be so; in special cases it may be otherwise, namely, when the advantage offered by parthenogenesis in respect to the maintenance of the species preponderates over the advantage which amphimixis implies as regards possibilities of transformation. As far as we have seen from the case of the gall-wasps, the absence of amphimixis in every second generation implies no disadvantage in regard to the capability for transformation which the species exhibits. As to whether any disadvantage would ensue if the number of parthenogenetic generations in the life-cycle were greater we can only guess, since no case is known which enables us to decide this point, _pro_ or _con_, with any certainty. The heterogony of the plant-lice, the Aphides, and their relatives might be cited as against the probability, for in this case a long series of parthenogenetic generations often alternates with a single bi-sexual one, but the difference in structure is not so great in this case, although it does exist, and moreover we can quite well assume that the adaptation to parthenogenesis was effected at the beginning of heterogony, when it still consisted of a cycle of only two generations, and that further virgin generations were interpolated subsequently. This assumption is supported by the fact that in some species of our indigenous Ostracods, in _Cypris vidua_ and _Candona candens_, in contrast to the Daphnids, several bi-sexual generations alternate with one parthenogenetic generation. But in this case again there is no difference whatever in the structure of the two generations, the parthenogenetic generation being distinguished from the bi-sexual generation simply by the absence of males. The alternation of generations in the plant-lice is particularly instructive, because it emphatically indicates how much Nature is concerned with the retention of amphimixis, and how little mere multiplication has to do with this. This is especially striking in the case of the bark-lice; for instance, in their notorious representative, the vine-pest, _Phylloxera vastatrix_. [Illustration: FIG. 127. Life-cycle of the Vine-pest (_Phylloxera vastatrix_), after Leuckart and Nitsche, and Ritter and Rübsamen. _A_, the fertilized ovum. _B_, the resulting apterous and parthenogenetic Phylloxera. _C_, its eggs, from which, as the uppermost arrow indicates, there may arise similar apterous, parthenogenetic forms, or, as the horizontal arrow indicates, winged forms (_D_), which produce 'female' and 'male' ova (_E^1_ and _E^2_); from these the sexual generation arises, the female (_F^1_) and the male (_F^2_); the former lays the fertilized ovum (_A_).]