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 the living of an organism, and the expressions “division of labor” and “differentiation” come to have a biological meaning and application.

* * * * *

The cell, then, is in all respects the very unit of the organic world.  Not only is it the ultimate structural element of all the more familiar animals and plants that we know, as the foregoing analysis demonstrates, but, in the second place, the microscope reveals simple little organisms, like Amoeba, the yeast plant and bacteria, which consist throughout their lives of just one cell and nothing more.  Still more wonderful is the fact that the larger complex organisms actually begin existence as single cells.  In three ways, therefore,—­the analytic, the comparative, and the developmental,—­the cell proves to be the “organic individual of the first order.”  As the ultimate biological unit, its essential nature must possess a profound interest, for in its substance resides the secret of life.

This wonderful physical basis of life is called protoplasm.  It contains three kinds of chemical compounds known as the proteins, carbohydrates, and hydrocarbons.  Proteins are invariably present in living cells, and are made up of carbon, hydrogen, nitrogen, sulphur, and usually a little phosphorus.  The elements are also combined in a very complex chemical way.  For example, the substance called haemoglobin is the protein which exists in the red blood cells and which causes those cells to appear light red or yellow when seen singly.  Its chemical formula states the precise number of atoms which enter into the constitution of a single molecule as:  C_{600}H_{960}N_{154}FeO_{179}.  This is truly a marvelously complex substance when compared with the materials of the inorganic world, like water, for example, which has the formula H_{2}O.  And just as the peculiar properties of H_{2}O are given to it by the properties of the hydrogen and the oxygen which combine to form it, just so, the scientist believes, the marvelous properties of protein are due to the assemblage of the properties of the carbon and hydrogen and other elements which enter into its composition.