Finally, in the last analysis, all organisms and organs and tissues can be resolved into elements which are called _cells_. They are not little hollow cases, it is true, although for historical reasons we employ a word that implies such a condition. They are unitary masses of living matter with a peculiar central body or nucleus, and every tissue of every living thing is composed of them. The cells of bone differ from those of cartilage mainly in the different consistency of the substances secreted by the cells to lie between them; skin cells are soft-walled masses lying close together; even blood is a tissue, although it is fluid and its cells are the corpuscles which float freely in a liquid serum. Thus an organism proves to be a complex mechanism composed of cells as structural units, just as a building is ultimately a collection of bricks and girders and bolts, related to one another in definite ways. Our analysis reveals the living creature in an entirely new light, not only as a machinelike structure whose parts are marvelously formed and coordinated in material respects, but also as one whose activities or workings are ultimately cellular in origin. Structure and function are inseparable, and if an animal or a plant is an aggregate of cells, then its whole varied life must be the sum total of the lives of its constituent cells. Should these units be subtracted from an animal, one by one, there would be no material organism left when the last cells had been disassociated, and there would be no organic activity remaining when the last individual cell-life was destroyed. All the various things we do in the performance of our daily tasks are done by the combined action of our muscle and nerve and other tissue cells; our life is all of their lives, and nothing more. The cell, then, is the physiological or functional unit, as truly as it is the material element of the organic world. Being combined with countless others, specialized in various ways, relations are established which are like those exhibited by the human beings constituting a nation. In this case the life of the community consists of the activities of the diverse human units that make it up. The farmer, the manufacturer, the soldier, clerk, and artisan do not all work in the same way; they undertake one or another of the economic tasks which they may be best fitted by circumstances to perform. Their differentiation and division of labor are identical with the diversity in structure and in function as well, exhibited by the cells of a living creature. We might speak of the several states as so many organs of our own nation; the commercial or farming or manufacturing communities of a state would be like the tissues forming an organ, made up ultimately of human units, which, like cells, are engaged in similar activities. As the individual human lives and the activities of differentiated economic groups constitute the life of a nation and national existence, so cell-lives make