Evolution establishes in them both, not differences simply, but definitely-connected differences—differences such that each makes the others possible. The parts of an inorganic aggregate are so related that one may change greatly without appreciably affecting the rest. It is otherwise with the parts of an organic aggregate or of a social aggregate. In either of these the changes in the parts are mutually determined, and the changed actions of the parts are mutually dependent. In both, too, this mutuality increases as the evolution advances. The lowest type of animal is all stomach, all respiratory surface, all limb. Development of a type having appendages by which to move about or lay hold of food can take place only if these appendages, losing power to absorb nutriment directly from surrounding bodies, are supplied with nutriment by parts which retain the power of absorption. A respiratory surface, to which the circulating fluids are brought to be aërated, can be formed only on condition that the concomitant loss of ability to supply itself with materials for repair and growth is made good by the development of a structure bringing these materials. So is it in a society. What we call with perfect propriety its organization has a necessary implication of the same kind. While rudimentary, it is all warrior, all hunter, all hut-builder, all tool-maker: every part fulfills for itself all needs. Progress to a stage characterized by a permanent army can go on only as there arise arrangements for supplying that army with food, clothes, and munitions of war, by the rest. If here the population occupies itself solely with agriculture and there with mining—if these manufacture goods while those distribute them—it must be on condition that, in exchange for a special kind of service rendered by each part to other parts, these other parts severally give due proportions of their services.
This division of labor, first dwelt on by political economists as a social phenomenon, and thereupon recognized by biologists as a phenomenon of living bodies, which they called the "physiological division of labor," is that which in the society, as in the animal, makes it a living whole. Scarcely can I emphasize sufficiently the truth that, in respect of this fundamental trait, a social organism and an individual organism are entirely alike. When we see that, in a mammal, arresting the lungs quickly brings the heart to a stand; that if the stomach fails absolutely in its office all other parts by-and-by cease to act; that paralysis of its limbs entails on the body at large death from want of food or inability to escape; that loss of even such small organs as the eyes deprives the rest of a service essential to their preservation—we cannot but admit that mutual dependence of parts is an essential characteristic. And when, in a society, we see that the workers in iron stop if the miners do not supply materials; that makers of clothes cannot carry on their business in the absence of those who spin and weave textile fabrics; that the manufacturing community will cease to act unless the food-producing and food-dis-
