Too much agitation—that is, too high a temperature—will split them up and destroy the new-found potentiality of such aggregates; too little agitation—that is, too low a temperature—will permit them to begin to cohere and settle down into frozen rigid masses insusceptible of manifold activities. But take them just at the right temperature, when sufficiently complex and sufficiently mobile; take care of them, so to speak, for the structure may easily be killed; and what shall we find? We could not infer or guess what would be the result, but we can observe the result as it is. The result is that the complexes group themselves into minute masses visible in the microscope, each mass being called by us a "cell"; that these cells possess the power of uniting with or assimilating other cells, or fragments of cells, as they drift by and come into contact with them; and that they absorb into their own substance such portions as may be suitable, while the insufficiently elaborated portions—the grains of inorganic or over-simple material—are presently extruded. They thus begin the act of "feeding." Another remarkable property also can be observed; for a cell which thus grows by feeding need not remain as one individual, but may split into two, or into more than two, which may cohere for a time, but will ultimately separate and continue existence on their own account. Thus begins the act of "reproduction." But a still more remarkable property can be observed in some of the cells, though not in all; they can not only assimilate a fragment of matter which comes into contact with them, but they can sense it, apparently, while not yet in contact, and can protrude portions of their substance or move their whole bodies towards the fragment, thus beginning the act of "hunting"; and the incipient locomotory power can be extended till light and air and moisture and many other things can be sought and moved towards, until locomotion becomes so free that it sometimes seems apparently objectless—mere restlessness, change for the sake of change, like that of human beings. The power of locomotion is liable, however, to introduce the cell to new dangers, and to conditions hostile to its continued aggregate existence. So, in addition to the sense of food and other desirable things ahead, it seems to acquire, at any rate when still further aggregated and more developed, a sense of shrinking from and avoidance of the hostile and the dangerous,—a sense as it were of "pain."