In his interesting book, already mentioned, Dr. Johnstone called attention to the power possessed by living matter of reversing the process; but no reversal of this kind and extent can make up for the constant degradation of energy which is taking place all round us. We mention this because it shows that "energy" cannot, in any case, afford an eternal solution, but only a temporal and therefore a limited one. No one doubts that there is energy in the living thing, nor that there are what the author calls "complexes of energies." No one, again, will quarrel with the statement that energy is first seen in the sun, in the earth, in the air, and in the water; that "with life something new appears in the universe, namely, a union of the internal and external adjustment of energy which we appropriately call an _Organism_." That "the germ is an energy complex" is no doubt an unproved hypothesis, as he admits, but is quite likely. With all these assertions we may agree, though we cannot with that which follows, namely, that energy is creative, for that such is impossible in any true sense of that word we have already tried to show. We have now to ask ourselves in what way this energy conception of life differs from, or goes beyond, the two theories of life--mechanistic and vitalistic, which have hitherto been supposed to have exhausted the possibilities of explanation. In order to do this we must analyse the author's idea of energy and its relationship to biological processes a little more closely. He begins his study of life and its evolution by considering how nutrition and the derivation of energy can have taken place before chlorophyl had come into existence; and he very pertinently points to the _prototrophic_ bacteria as probably representing "the survival of a primordial stage of life chemistry." Thus a "primitive feeder," the bacterium _Nitrosomonas_, "for combustion ... takes in oxygen directly through the intermediate action of iron, phosphorus or manganese, each of the single cells being a powerful little chemical laboratory which contains oxidising catalysers, the activity of which is accelerated by the presence of iron and manganese. Still, in the primordial stage, _Nitrosomonas_ lives on ammonium sulphate, taking its energy (food) from the nitrogen of ammonium and forming nitrates. Living symbiotically with it is _Nitrobacter_, which takes its energy (food) from the nitrates formed by _Nitrosomonas_, oxidising them into nitrates. Thus these two species illustrate in its simplest form our law of the _interaction of an organism_ (_Nitrobacter_) _with its life environment_ (_Nitrosomonas_)" (p. 82, author's italics).