All living organisms consist, then simply of cells. Those consisting of but one cell are termed unicellular; those comprising more than one cell are called pluricellular. 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. [Sidenote: Mind Life of One Cell] 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." [Sidenote: The Will of the Cell] 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. [Sidenote: The Cell and Organic Evolution] 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, Verwörn 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 pluricellular organisms and consider them particularly from the standpoint of organic evolution. The pluricellular organism is nothing more nor less than a later development, a confederated association of unicellular organisms. Mark the development of such an association. [Sidenote: Evolutionary Differentiation] 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.