been the living machine as we know it in its highest form, with its numerous organs, all interrelated in such a way as to form a harmoniously acting whole. [Illustration: FIG. 46. A later step in machine building in which the outer cells have acquired different form and function from the inner cells: _ec_, the outer cells, whose duties are protective; _en_, the inner cells engaged in digesting food.] But a second principle in this growth of the machine was needed to produce the variety which is found in nature. As the different cells in the multicellular mass became associated into groups for different duties, the method of such division of labor was not alike in all machines. A city in China and one in America are alike made up of individuals, and the fundamental needs of the Chinaman and the American are alike. But differences in industrial and political conditions have produced different combinations and associations, so that Pekin is wonderfully unlike New York. So in these early developing machines, quite a variety of method of organization was adopted by the different groups. Now as soon as any special type of organization was adopted by any animal or plant, the principle of heredity transmitted the same kind of organization to its descendants, and there thus arose lines of descent differing from each other, each line having its own method of organization. As we follow the history of each line the same thing is repeated. We find that the representatives of each line again separate into groups, each of which has acquired some new type of organization, and there has thus been a constant divergence of these lines of descent in an indefinite number of directions. The members of the different lines of descent all show a fundamental likeness with each other since they retain the fundamental characters of their common ancestor, but they show also the differences which they have themselves acquired. And thus the process is repeated over and over again. This history of the growth of these different machines has thus been one of divergence from common centres, and is to be diagrammatically expressed after the fashion of a branching tree. The end of each branch represents the highest state of perfection to which each line has been carried.