There are two kinds of homogenesis, the simplest of them, probably once universal but now exceptional, being that in which there is no other form of multiplication than one resulting from perpetual spontaneous fission. The rise of distinct sexes was doubtless a step in evolution, and before it took place the formation of new individuals could have arisen only by division of the old, either into two or into many. At present this process survives, so far as appears, among _Bacteria_, certain _Algæ_, and sundry _Protozoa_; though it is possible that a rarely-occurring conjugation has in these cases not yet been observed. It is a probable conclusion, however, that in the _Bacteria_ at any rate, the once universal mode of multiplication still survives as an exceptional mode. But now passing over these cases, we have to note that the kind of genesis (once supposed to be the sole kind), in which the successive generations are alike, is sexual genesis, or, as it has been otherwise called--_gamogenesis_. In every species which multiplies by this kind of homogenesis, each generation consists of males and females; and from the fertilized germs they produce the next generation of similar males and females arises: the only needful qualification of this statement being that in many _Protophyta_ and _Protozoa_ the conjugating cells or protoplasts are not distinguishable in character. This mode of propagation has the further trait, that each fertilized germ usually gives rise to but one individual--the product of development is organized round one axis and not round several axes, Homogenesis in contrast with heterogenesis as exhibited in species which display distinct sexuality, has also the characteristic that each new individual begins as an egg detached from the maternal tissues, instead of being a portion of protoplasm continuous with them, and that its development proceeds independently. This development may be carried on either internally or externally; whence results the division into the oviparous and the viviparous. The oviparous kind is that in which the fertilized germ is extruded from the parent before it has undergone any considerable development. The viviparous kind is that in which development is considerably advanced, or almost completed, before extrusion takes place. This distinction is, however, not a sharply-defined one: there are transitions between the oviparous and the viviparous processes. In ovo-viviparous genesis there is an internal incubation; and though the young are in this case finally extruded from the parent in the shape of eggs, they do not leave the parent's body until after they have assumed something like the parental form. Looking around, we find that homogenesis is universal among the _Vertebrata_. Every vertebrate animal arises from a fertilized germ, and unites into its single individuality the whole product of this fertilized germ. In the mammals or highest _Vertebrata_, this