as every one cannot work at the same trade, and the dependence of one upon another becomes very great. This is immediately seen if we consider the confusion that would arise in a city if the butchers and bakers were suddenly to die. But the community in general is not only differentiated by the Struggle for Existence into divers interests, but, sooner or later, the individuals are affected in the same way. For the individual whose organization is most specialized and whose functions are many is better fitted to maintain himself against the changing conditions of life than one whose organization is more simple. But we have seen that one variation entails another, and that the peculiarities of the parents are transmitted to their offspring: hence in the course of generations the organization becomes extremely complex. Thus the Division of Labor is carried out to such an extent in the organization of the human body that it requires volumes to describe the anatomy of man. The Division of Labor, or a complex organization, does not necessarily follow from the Struggle for Existence; for there are animals who, when young, lead a free active life, and have quite a complex organization, but, growing older, they adopt a parasitic mode of life, and then lose many of their organs through disuse. Prof. Haeckel aptly observes, "The traveler lightens his journey who throws away his pack." So of many parasites: the one who first gets rid of any useless muscles or nerves will have the best chance of surviving; complex conditions of existence bring about, sooner or later, complex organization, while simple structures are the result of simple conditions of existence. Supposing this view of Nature to be correct, the plants and animals that first appeared on the earth ought to have been simply and lowly organized, the later ones highly complex. In our chapter on Geology we have shown that such is the case,—that there has been a progress from the lower to the higher forms of life, accompanied at the
