§ 202. A truth which we before saw among plants, we here see repeated among animals--the truth that as soon as the integration of aggregates of the first order into aggregates of the second order, produces compound wholes so specific in their shapes and sizes, and so mutually dependent in their parts, as to have distinct individualities; there simultaneously arises the tendency in them to produce, by gemmation, other such aggregates of the second order. The approach towards definite limitation in an organism, is, by implication, an approach towards a state in which growth passing a certain point, results, not in the increase of the old individual, but in the formation of a new individual. Thus it happens that the common polype buds out other polypes, some of which very shortly do the like, as shown in Fig. 148: a process paralleled by the fronds of sundry _Algæ_, and by those of the lower _Jungermanniaceæ_. And just as, among these last plants, the proliferously-produced fronds, after growing to certain sizes and developing rootlets, detach themselves from their parent fronds; so among these animals, separation of the young ones from the bodies of their parents ensues when they have acquired tolerably complete organizations. [Illustration: Figs. 148–150.] There is reason to think that the parallel holds still further. Within the limits of the _Jungermanniaceæ_, we found that while some genera exhibit this discontinuous development, other genera exhibit a development that is similar to it in all essential respects, save that it is continuous. And here within the limits of the _Hydrozoa_, we find, along with this genus in which the gemmiparous individuals are presently cast off, other genera in which they are not cast off, but form a permanent aggregate of the third order. Figs. 149 and 150, exemplify these compound _Hydrozoa_--one of them showing this mode of growth so carried out as to produce a single axis; and the other showing how, by repetitions of the process, lateral axes are produced. Integrations characterizing certain higher genera of the _Hydrozoa_ which swim or float instead of being fixed, are indicated by Figs. 151 and 152: the first of them representing the type of a group in which the polypes growing from an axis, or cœnosarc, are drawn through the water by the rhythmical contractions of the organs from which they hang; and the second of them representing a _Physalia_ the component polypes of which are united into a cluster, attached to an air-vessel. [Illustration: Figs. 151–152.]