§ 211. On glancing back at the facts brought together in these two chapters, we see it to be probable that there has gone on among animals a process like that which we saw reason to think has gone on among plants. Minute aggregates of those physiological units which compose living protoplasm, exist as _Protozoa_: some of them incoherent, indefinite, and almost homogeneous, and others of them more coherent, definite, and heterogeneous. By union of these nucleated particles of sarcode, are produced various indefinite aggregates of the second order--Sponges, _Polycytharia_, Foraminifers, &c.; in which the compound individuality is scarcely enough marked to subordinate the primitive individualities. But in other types, as in _Hydra_, the lives of the morphological units are in a considerable degree, though not wholly, merged in the life of the integrated body they form. As the primary aggregate, when it passes a certain size, undergoes fission or gemmation; so does the secondary aggregate. And as on the lower stage so on the higher, we see cases in which the gemmiparously-produced individuals part as soon as formed, and other cases in which they continue united, though in great measure independent. This massing of secondary aggregates into tertiary aggregates, is variously carried on among the _Hydrozoa_, the _Actinozoa_, the _Polyzoa_, and the _Tunicata_. In most of the types so produced, the component individualities are very little subordinated to the individuality of the composite mass--there is only physical unity and not physiological unity; but in certain of the oceanic _Hydrozoa_, the individuals are so far differentiated and combined as very much to mask them. Forms showing us clearly the transition to well-developed individuals of the third order, are not to be found. Nevertheless, in the great sub-kingdom _Annulosa_, there are traits of structure, development, and mode of multiplication, which go far to show that its members are such individuals of the third order; and in the relations to external conditions involved by the mode of union, we find an adequate cause for that obscuration of the secondary individualities which we must suppose has taken place. The two other great subdivisions, _Mollusca_ and _Vertebrata_, between the lower members of which there are suggestive points of community, present us only with aggregates of the second order, that have in many cases become very large and very complex. We find in them no trace of the union of gemmiparously-produced individuals. Neither the molluscous nor the vertebrate animal shows the faintest trace of a segmentation affecting the totality of its structure; and we see good grounds for concluding that such segmentation as exceptionally occurs in the one and usually occurs in the other, is superinduced. * * * * *