is but one step to the conception of gemmiparously-produced individuals which severally part one from another as soon as they are formed. § 209. We must now return to a junction whence we diverged some time ago. As before explained under the head of Classification, organisms do not admit of uniserial arrangement, either in general or in detail; but everywhere form groups within groups. Hence, having traced the phases of morphological composition up to the highest forms in any sub-kingdom, we find ourselves at the extremity of a great branch, from which there is no access to another great branch, except by going back to some place of bifurcation low down in the tree. There exist such similarities of shape and structure between the larval forms of low Molluscs and those of Annelids and Rotifers, as to show that there was an early type common to them all; and its probable characters, suggested by comparison, seem to imply that it had arisen from some cœlenterate type, intermediate between the _Cnidaria_ and the _Ctenophora_. But there is this noteworthy difference between the molluscan larva and the allied larvæ, that it gives origin to only one animal and not to a group of animals, united or disunited. No true Mollusc multiplies by gemmation, either continuous or discontinuous; but the product of every fertilized germ is a single individual. It is a significant fact that here, where for the first time we have homogenesis holding throughout an entire sub-kingdom, we have also throughout an entire sub-kingdom no case in which the organism is divisible into two, three, or more, like parts. There is neither any such clustering or branching as a cœlenterate or molluscoid animal usually displays; nor is there any trace of that segmentation which characterizes the _Annulosa_. Among these animals in which no single egg produces several individuals, no individual is separable into several homologous divisions. This connexion will be seen to have a probable meaning, on remembering that it is the converse of the connexion which obtains among the _Annulosa_, considered as a group. A Mollusc, then, is an aggregate of the second order. Not only in the adult animal is there no sign of a multiplicity of like parts that have become obscured by integration; but there is no sign of such multiplicity in the embryo. And this unity is just as conspicuous in the lowest Lamellibranch as in the highest Cephalopod. [Illustration: Figs. 188–190.]