To take a less trivial instance; a planet which is large enough to retain an atmosphere by its gravitative attraction differs utterly, in potentiality and importance, from the numerous lumps of matter scattered throughout space, which, though they may be as large as a haystack or a mountain or as the British Isles, or even Europe, are yet too small to hold any trace of air to their surface, and therefore cannot in any intelligible sense of the word be regarded as habitable. One of the lumps of matter in space can become a habitable planet only when it has attained a certain size, which conceivably it might do by falling together with others into a complex aggregate under the influence of gravitative attraction. The asteroids have not succeeded in doing this, but the planets have; and, accordingly, one of them, at any rate, has become a habitable world. 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.