Meanwhile, the great laws of "adaptation" and "heredity," the basic principles of evolution, have been steadily at work, and slowly there has come about a differentiation of cell function, an apportionment among the different cells of the different kinds of labor. [Sidenote: Plurality of the Individual] As the result of such differentiation, the pluricellular organism, as it comes ultimately to be evolved, is composed of many different kinds of cells. Each has its special function. Each has its field of labor. Each lives its own individual life. Each reproduces its own kind. Yet all are bound together as elements of the same "cell society" or organized "cell state." Among pluricellular organisms man is of course supreme. He is the one form of animal life that is most highly differentiated. [Sidenote: Combined Consciousness of the Millions] Knowing what you now know of microscopic anatomy, you cannot hold to the simple idea that the human body is a single life-unit. This is the naïve belief that is everywhere current among men today. Inquire among your own friends and acquaintances and you will find that not one in a thousand realizes that he is, to put it jocularly, singularly plural, that he is in fact an assemblage of individuals. [Illustration: MICROSCOPIC STUDIES IN HUMAN ANATOMY, PRIVATE LABORATORY, SOCIETY OF APPLIED PSYCHOLOGY] Not only is the living human body as a whole alive, but "every part of it as large as a pin-point is alive, with a separate and independent life all its own; every part of the brain, lungs, heart, muscles, fat and skin." No man ever has or ever can count the number of these parts or cells, some of which are so minute that it would take thousands in a row to reach an inch. "Feeling" or "consciousness" is the sum total of the feelings and consciousness of millions of cells, just as an orchestral harmony is a composite of the sounds of all the individual instruments. [Sidenote: Evolution of the Human Organism] In the ancient dawn of evolution, all the cells of the human body were of the same kind. But Nature is everywhere working out problems of economy and efficiency. And, to meet the necessities of environment, there has gradually come about a parceling out among the different cells of the various tasks that all had been previously called upon to perform for the support of the human institution. This differentiation in kinds of work has gradually brought about corresponding and appropriate changes of structure in the cells themselves, whereby each has become better fitted to perform its part in the sustenance and growth of the body. [Sidenote: The Crowd-Man] When you come to think that these processes of adaptation and heredity in the human body have been going on for _countless millions of years_, you can readily understand how it is that the human body of today is made up of more than thirty different kinds of cells, each having its special function. [Sidenote: Functions of Different Human Cells]