But observe that the great size and the consequent retention of an atmosphere did not generate the inhabitants; it satisfied one of the conditions necessary for their existence. How they arose is another matter. All that we have seen so far is that an aggregate of bodies may possess properties and powers which the separate bodies themselves possess in no kind or sort of way. It is not a question of degree, but of kind.
So also, further, if the aggregate is large enough, very much larger than any planet, as large as a million earths aggregated together, it acquires the property of conspicuous radio-activity, it becomes a self-heating and self-luminous body, able to keep the ether violently agitated in all space round it, and thus to supply the radiation necessary for protecting the habitable worlds from the cold of space to which they are exposed, for maintaining them at a temperature appropriate to organic existence, and likewise for supplying and generating the energy for their myriad activities. It has become in fact a central sun, and source of heat, solely because of its enormous size combined with the fact of the mutual gravitative attraction of its own constituent particles. No body of moderate size could perform this function, nor act as a perennial furnace to the rest.
Application to Protoplasm.
Very well then, return now to our complex molecular aggregate, and ask what new property, beyond the province of ordinary chemistry and physics, is to be expected of a compound which contains millions or billions of atoms attached to each other in no rigid, stable, frigid manner, but by loose unstable links, enabling them constantly to re-arrange themselves and to be the theatre of perpetual change, aggregating and reaggregating in various ways and manifesting ceaseless activities. Such unstable aggregates of matter may, like the water of a pond or a heap of organic refuse, serve as the vehicle for influences wholly novel and unexpected.
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."
And so it enters on its long career of progress, always liable to disintegration or "death"; it begins to differentiate portions of itself for the feeding process, other portions for the reproductive process, other portions again for sensory processes, but retaining the protective sense of pain almost everywhere; until the spots sensitive to ethereal and aerial vibrations—which, arriving as they do from a distance, carry with them so much valuable information, and when duly appreciated render possible perception and prediction as to what is ahead—until these sensitive spots have become developed into the special organs which we now know as the "eye" and the "ear." Then, presently, the power of communication is slowly elaborated, speech and education begin, and the knowledge of the individual is no longer limited to his own experience, but expands till it embraces the past history and the condensed acquisition of the race. And thus gradually arises a developed self-consciousness, a discrimination between the self and the external world, and a realisation of the power of choice and freedom,—a stage beyond which we have not travelled as yet, but a stage at which almost all things seem possible.