themselves producing fertilized ova, produce, by gemmation, strings of individuals from which fertilized ova again originate. In multiaxial plants, we have a succession of generations represented by the series A, B, B, B, &c., A, B, B, B, &c. Supposing A to be a flowering axis or sexual individual, then, from any fertilized germ it casts off, there grows up a sexless individual, B; from this there bud-out other sexless individuals, B, and so on for generations more or less numerous, until at length, from some of these sexless individuals, there bud-out seed-bearing individuals of the original form A. Branched herbs, shrubs, and trees, exhibit this form of heterogenesis: the successive generations of sexless individuals thus produced being, in most cases, continuously developed, or aggregated into a compound individual, but being in some cases discontinuously developed. Among animals a kind of heterogenesis represented by the same succession of letters, occurs in such compound polypes as the _Sertularia_, and in those of the _Hydrozoa_ which assume alternately the polypoid form and the form of the _Medusa_. The chief differences presented by these groups arise from the fact that the successive generations of sexless individuals produced by budding, are in some cases continuously developed, and in others discontinuously developed; and from the fact that, in some cases, the sexual individuals give off their fertilized germs while still growing on the parent-polypedom, but in other cases not until after leaving the parent-polypedom and undergoing further development. Where, as in all the foregoing kinds of agamogenesis, the new individuals bud out, not from any specialized reproductive organs but from unspecialized parts of the parent, the process has been named, by Prof. Owen, _metagenesis_. In most instances the individuals thus produced grow from the outsides of the parents--the metagenesis is external. But there is also a kind of metagenesis which we may distinguish as internal. Certain _entozoa_ of the genus _Distoma_ exhibit it. From the egg of a _Distoma_ there results a rudely-formed creature known as a sporocyst and from this a redia. Gradually, as this divides and buds, the greater part of the inner substance is transformed into young animals called _Cercariæ_ (which are the larvæ of _Distomata_); until at length it becomes little more than a living sac full of living offspring. In the _Distoma pacifica_, the brood of young animals thus arising by internal gemmation are not _Cercariæ_, but are like their parent: themselves becoming the producers of _Cercariæ_, after the same manner, at a subsequent period. So that now the succession of forms is represented by the series A, B, A, B, &c., now by the series A, B, B, A, B, B, &c., and now by A, B, B, C, A. Both cases, however, exemplify internal metagenesis in contrast with the several kinds of external metagenesis described above. That agamogenesis which is carried on