780 THE AMERICAN JOURNAL OF SOCIOLOGY
not exist in plants. In the higher protozoa the pseudopods of the amoeba are represented by cilia, and the ectoderm of the lower invertebrates is generally ciliated in order to assist locomotion, as is the endoderm to promote the movement of food toward the digestive cavity. In the course of evolution the cilia of the ectoderm and of the endoderm have disappeared, and have been replaced by muscular movements.
What is it, then, from the point of view of limits, that dis- tinguishes the superior from the inferior forms ? The distinction consists in the increasing differentiation of functions and organs in the whole structure. The result is that the motility of animals augments with increasing complexity and co-ordination of structure. The superior animals move more rapidly and in more directions ; they do not have absolutely fixed limits ; they search for their prey and avoid their enemies. This difference in motor capacity is related to the difference in the development of their digestive, vascular, and respiratory organs, but princi- pally to their contractile structure, that is to say, their muscular system.
From the point of view of general philosophy, every struc- tural differentiation is the morphological aspect of a differentia- tionin the internal and external movement, which, constituting the life and equilibration of the structures may fall back to the most general laws of mechanics, although biology supposes special phenomena, which, like those of sociology, call for a particular interpretation and philosophy.
The muscular system is the fundamental structure making possible the expansions and contractions of organisms in a word, the performance of the movements appropriate to the milieu. But without the nervous system the muscles themselves would remain passive ; the real generator of motility then, is the nervous system ; it transmits the excitations from within and from without, and co-ordinates the movements of more and more special adaptations ; it maintains the general equilibrium in the midst of a more and more intense instability ; it is the regulator par excellence of organic structures. It is, then, only in appearance that organic bodies seem indeterminate in their
