We know, however, that life displays the same kind of uniformity that characterises heat, light, motion and other imponderables. The something that appears as the temperature of boiling water is similar in all respects to the something that appears as the equal temperature of hot oil and can be interchanged therewith. Indeed, the fundamental Theory of Exchanges upon which a great part of thermodynamics is based depends for its validity on the absence of any distinction between the heats of various masses of matter. The only variation of heat is that of degree: the kind is always the same; heat never becomes individualised. This is seen by everyday observation to be equally true of life, and is frequently demonstrated by specific experiment. Grafting, for example, whether it consist in the union of a scion of one tree with the stock of another, or whether it take the form of transferring a piece of John Smith’s skin to a flayed part of Robert Green’s arm, is the migration of a vitality that remains unchanged in spite of the change of environment and that intermingles harmoniously and homogeneously with the vitality of its new abode. Neither the individualities of the two trees nor the personalities of the two men appear in, or accompany, their stocks of “life” any more than they are to be found in their stocks of heat or weight. The various stocks may be more or less abundant in quantity, but they do not differ in kind. We know also that life is capable of indefinite increase by reproduction, provided only that the means of sustenance be available. A single pair of rabbits, for instance, if allowed to breed unchecked and not killed off, will, in a comparatively short time, become represented by two millions of similar animals. This means, of course, that the quantity of “life” corresponding to two rabbits has been augmented a million-fold. The increase has not been derived from the food consumed, the total amount of which is accounted for by the bodies and excreta of the conies. A similar phenomenon is observable throughout the whole sum of living beings, whether human, “animal” or vegetable. It distinguishes “life” very effectually from matter and energy, both of which are, by the doctrine of conservation, as incapable of increase as of decrease. The only hypothesis that appears possible by way of explanation is to hold that “life” is one of the protean modes of energy in the same way that heat is understood (by those persons who are content to accept the Baconian, and modern scientific view) to be a mode of motion. This hypothesis, however, does not rest on any secure foundation. The only energy that is known to be practically available for transmutation into life is heat (light and electricity seem to be negligible); and reproduction, which often takes place on a very large scale, has never been observed to involve the absorption and disappearance of heat.