Under heterogenesis then is comprised, not only as heretofore the reproduction of _Ascaris nigrovenosa_, of _Leptodora appendiculata_, and of the cattle-lice, but also that of the _Aphides_, _Coccidæ_, _Daphniidæ_, _Rotatoria_, and _Phyllopoda_, and, in short, all those cases in which we can determine the former identity of the two kinds of generations from their form, anatomical structure, and mode of reproduction. This conclusion is essentially supported by a comparison of the most closely allied species. Thus, for instance, when we see the genus _Aphis_ and its allies related on all sides to insects which propagate sexually in all generations, and when we further observe the great similarity of the whole external and internal structure in the two kinds of generations of _Aphis_, we are forced to the conjecture that the apparent asexual reproduction of the _Aphidæ_ is in reality parthenogenesis, i.e., that it has been developed from sexual reproduction. Neither can it be any longer disputed that in this case, as well as in that of _Leptodora_ and other _Daphniidæ_, the same female alternately propagates parthenogenetically, and produces eggs requiring fertilization. This was established by Von Heyden[49] some years ago, in the case of _Lachnus Querci_, and has been since confirmed by Balbiani.[50] There can be no doubt that in all these cases the cycle of generations has been developed from phyletically similar generations. But instances are certainly conceivable which present themselves with less clearness and simplicity. In the first place, we do not know whether parthenogenesis may not finally settle down into complete asexual reproduction. Should this be the case, it might be possible that from heterogenesis a mode of propagation would ultimately arise, which was apparently indistinguishable from pure metagenesis. Such a state of affairs might result, if the generations settling into asexual reproduction (as, for instance, the plant-lice), at the same time by adaptation to varying conditions of life, underwent considerable change of structure, and entered upon a metamorphosis to some extent retrogressive. We should then be inclined to regard these generations as an earlier phyletic stage, whilst, in fact, they would be a later one, and the idea of metagenesis would thus have been formed after the manner of heterogenesis.