§ 242. The general considerations which preluded our inquiry into the shapes of plants and their parts, equally serve, so far as they go, to prelude an inquiry into the shapes of animals and their parts. Among animals, as among plants, the formation of aggregates greater in bulk or higher in degree of composition, or both, is accompanied by changes of form in the aggregates as wholes as well as by changes of form in their parts; and the processes of morphological differentiation conform to the same general laws in the one kingdom as in the other. It is needless to recapitulate the several kinds of modification to be explained, and the several factors that co-operate in working them. In so far as these are common to plants and animals, the preceding chapters have sufficiently familiarized them. Nor is it needful to specify afresh the several types of symmetry and their descriptive names; for what is true of them in the one case is true of them in the other. There is, however, one new and all-important factor which we shall have now to take into account; and about this a few preliminary remarks are requisite. § 243. This new factor is motion--motion of the organism in relation to surrounding objects, or of the parts of the organism in relation to one another, or both. Though there are plants, especially of the simpler kinds, which move, and though a few of the simpler animals do not move; yet movements are so exceptional and unobtrusive in the one kingdom, while they are so general and conspicuous in the other, that the broad distinction commonly made is well warranted. What, among plants, is an inappreciable cause of morphological differentiation, becomes, among animals, the chief cause of morphological differentiation. Rooted animals or animals otherwise fixed, of course present traits of structure nearest akin to those we have lately been studying. The motions of parts in relation to one another and to the environment, being governed by the mode of aggregation and mode of fixing, we are presented with morphological differentiations similar in their general characters to those of plants, and showing us parallel kinds of symmetry under parallel conditions. But animals which move from place to place are subject to an additional class of actions and reactions. These actions and reactions affect them in various ways according to their various modes of movement. Let us glance at the several leading relations between shape and motion which we may expect to find.