834 THE AMERICAN JOURNAL OF SOCIOLOGY
equilibrium. At the same time, we see how, as soon as an agglomeration of cellules is formed and limited by a superficies, some of these cellules those that are superficial and directly in contact with the surroundings tend to be differentiated and to play the role of officers of foreign affairs.
This primary differentiation is the expression of the constant changes taking place between the interior and the exterior. It is this continual movement which, incessantly renewing living matter, constitutes life. The form of a living body appears to us then, not as predetermined, but as resulting from the equilibrium of its forces or properties in interaction with the exterior forces or properties.
The materials and organic forces, as well as their forms, being limited, we have seen that their possible variations, though innumerable, are also limited. The variability is not absolute, but indefinite. This is true of the human species ; its physical and mental properties are circumscribed by relative maxima and relative minima. This limitation has been observed in pathol- ogy of the mind (suicide and lunacy).
After having set forth the constant limitations of the natural forces considered in their abstract elements, we have recognized the same law in aggregates where these forces are realized. This limitation is manifested in the distribution of climates, in that of geological layers and minerals, in geographic, orographic, and hydrographic distribution. It holds in the distribution of flora and of fauna. We have observed veritable empires and king- doms, and other and smaller divisions, with clearly defined boundaries, which are always determined by the great law of equilibrium of internal forces interacting with external forces. Finally, we have met with the distribution of the human race. We have seen that its divisions into varieties and races has been the natural process by which it has succeeded in adapting itself, better than any other species, to the most diverse conditions, but always within certain limits, which are those of that condition of life. The great variability of the human species has been the condition of its progress. Its differentiation into races and sub- races has facilitated the colonization of the earth, but, in return,
