The unicellular organism is the unit of life on this earth. Yet tiny and ultimate as it is, every unicellular organism is possessed of an independent and "free living" existence.

To be convinced of this fact, just consider for a moment the scope of development and range of activities of one of these tiny bodies.

"We see, then," says Haeckel, "that it performs all the essential life functions which the entire organism accomplishes. Every one of these little beings grows and feeds itself independently. It assimilates juices from without, absorbing them from the surrounding fluid. Each separate cell is also able to reproduce itself and to increase. This increase generally takes place by simple division, the nucleus parting first, by a contraction round its circumference, into two parts; after which the protoplasm likewise separates into two divisions. The single cell is able to move and creep about; from its outer surface it sends out and draws back again finger-like processes, thereby modifying its form. Finally, the young cell has feeling, and is more or less sensitive. It performs certain movements on the application of chemical and mechanical irritants."

The single living cell moves about in search of food. When food is found it is enveloped in the mass of protoplasm, digested and assimilated.

The single cell has the power of choice, for it refuses to eat what is unwholesome and extends itself mightily to reach that which is nourishing.

Moebius and Gates are convinced that the single cell possesses memory, for having once encountered anything dangerous, it knows enough to avoid it when presented under similar circumstances. And having once found food in a certain place, it will afterwards make a business of looking for it in the same place.

And, finally, Verworn and Binet have found in a single living cell manifestations of the emotions of surprise and fear and the rudiments of an ability to adapt means to an end.

Let us now consider pericellular organisms and consider them particularly from the standpoint of organic evolution. The pericellular organism is nothing more nor less than a later development, a confederated association of unicellular organisms. Mark the development of such an association.

Originally each separate cell performed all the functions of a separate life. The bonds that united it to its fellows were of the most transient character. Gradually the necessities of environment led to a more and more permanent grouping, until at last the bonds of union became indissoluble.

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.

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 pericellular organisms man is of course supreme. He is the one form of animal life that is most highly differentiated.

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 naive 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.

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.

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.

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.

We have muscle cells, with long, thin bodies like pea-pods, who devote their lives to the business of contraction; thin, hair-like connective tissue cells, whose office is to form a tough tissue for binding the parts of the body together; bone cells, a trades-union of masons, whose life work it is to select and assimilate salts of lime for the upkeep of the joints and framework; hair, skin, and nail cells, in various shapes and sizes, all devoting themselves to the protection and ornamentation of the body; gland cells, who give their lives, a force of trained chemists, to the abstraction from the blood of those substances that are needed for digestion; blood cells, crowding their way through the arteries, some making regular deliveries of provisions to the other tenants, some soldierly fellows patrolling their beats to repel invading disease germs, some serving as humble scavengers; liver cells engaged in the menial service of living off the waste of other organs and at the same time converting it into such fluids as are required for digestion; windpipe and lung cells, whose heads are covered with stiff hairs, which the cell throughout its life waves incessantly to and fro; and, lastly, and most important and of greatest interest to us, brain and nerve cells, the brain cells constituting altogether the organ of objective intelligence, the instrument through which we are conscious of the external world, and the nerve cells serving as a living telegraph to relay information, from one part of the body to another, with the "swiftness of thought."