§ 182. The transition to higher forms begins in a very unobtrusive manner. Among these aggregates of the first order, an approach towards that union by which aggregates of the second order are produced, is indicated by mere juxtaposition. Protophytes multiply rapidly; and their rapid multiplication sometimes causes crowding. When, instead of floating free in the water, they form a thin film on a moist surface, or are imbedded in a common matrix of mucilage; the mechanical obstacles to dispersion result in a kind of feeble integration, vaguely shadowing forth a combined group. Somewhat more definite combination is shown us by such plants as _Palmella botryoides_. Here the members of a family of cells, arising by the spontaneous fission of a parent-cell, remain united by slender threads of that jelly-like substance which envelops their surfaces. In some _Diatomaceæ_ several individuals, instead of completely separating, hold together by their angles; and in other _Diatomaceæ_, as the _Bacillaria_, a variable number of units cohere so slightly, that they are continually moving in relation to one another. This formation of aggregates of the second order, faintly indicated in feeble and variable unions like the above, may be traced through phases of increasing permanence and definiteness, as well as increasing extent. In the yeast-plant, Fig. 7, we have cells which may exist singly, or joined into groups of several; and which have their shapes scarcely at all modified by their connexion. Among the _Desmidiaceæ_, it happens in many cases that the two individuals produced by division of a parent-individual, part as soon as they are fully formed; but in other cases, instead of parting they compose a group of two. Allied kinds show us how, by subsequent fissions of the adherent individuals and their progeny, there result longer groups; and in some species, a continuous thread of them is thus produced. Figs. 8, 9, 11, exhibit these several stages. Fig. 10 represents a _Scenedesmus_ in which the individuation of the group is manifest. Instead of linear aggregation, many protophytes illustrate central aggregation; as shown in Figs. 12, 13, 14, 15. Other instances are furnished by such forms as the _Gonium pectorale_, Fig. 16 (_a_ being the front view, and _b_ the edge view), and the _Sarcina ventriculi_, Fig. 17. Further, we have that spherical mode of aggregation of which the _Volvox globator_ furnishes a familiar instance. [Illustration: Figs. 7–17.] [Illustration: Figs. 18–23.]