Carrying the analysis of organic structure one step further, it is found that the various organisms are themselves complex, being composed of tissues.  A frog’s leg as an organ of locomotion is composed of the protecting skin on the outside, the muscles, blood vessels, and nerves below, and in the center the bony supports of the whole limb.  Like the organs, these tissues are differentiated, structurally and functionally, and they also are so placed and related as to exhibit the kind of mechanical adjustment which we call adaptation.  The tissues, then, in their relations to the organs are like the organs in their relations to the whole creature, i.e. adapted to specific situations where they may most satisfactorily perform their tasks.

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,