INTRODUCTION TO SOCIOLOGY 781
structures and movements ; the truth is that, as one ascends the hierarchy of organisms, the more considerable variations are limited as much within as without by the more special and energetic agents of equilibration. The organs of life always determine and limit the structure and life.
However, at the basis of all organic life, under the names of contractility, irritability, sensibility, etc., we will always find motion; and so again, from the point of view of general philoso- phy, life can be expressed in terms of motion and in the laws of mechanics, themselves capable of being expressed mathe- matically.
It is only in appearance that the inferior social aggregates seem to have more fixed limits. In reality their movements are few and simple ; they are determined strictly by the simple gen- eral conditions of their external structure in relation to the most simple and general conditions of the environment, whether they are exclusively physical or both physical and social. A bad harvest, poor fishing, the disappearance of game, sometimes involve migrations, but more often still a partial or even total extinction of the horde, unless the aggregate is capable of undergoing the variations necessary to adapt itself to the new conditions or to modify them. In the latter case there will be formed a favorable variation in structure an increase of activ- ity or an extension of the agricultural, hunting, or fishing ter- ritory; or, indeed, certain classes of individuals will devote themselves to a new kind of work, while the others will continue to drag along in the old occupations. Often the increase will be, as in inorganic matter, by external growth, by addition; ordi- narily, however, there will be an incorporation, an assimilation giving place to an elaboration and to an internal development.
It is equally necessary to note that at the origin the external membrane is not very distinct from the milieu; it has not become strong as in the more developed stages. But that which char- acterizes the superior animals is that the internal organization gets the upper hand, where, for example, the shell, as in the carapace of the fish ganoideus (sturgeon) , gives place to an internal structure, frame, vertebral column, and organs. It is in the