In the state of Nature, vegetation dies and is left on the soil. Two things may then happen. It may become drawn into the soil by earthworms and other agents; the energy supply is thus distributed in the soil to serve the needs of the varied soil population. This is the normal case, associated with the normal soil population and the normal flora. If, however, the mingling agents are absent, the dead vegetation lies like a mat on the surface of the soil, only partially decomposing, unsuitable for the growth of most seedlings, and effectually preventing most of the vegetation below from pushing a way through: thus there comes to be no vegetation at all, or only a very restricted and special flora. The soil population becomes also specialised. Peats and acid grassland afford examples. On the neutral grass plots at Rothamsted, the dead vegetation does not accumulate on the surface but is rapidly decomposed or drawn into the soil, leaving the surface of the earth bare and free for the growth of seedlings. On the acid plots dead vegetation remains long on the surface, blotting out all new growth excepting two or three grasses which form underground runners capable of penetrating the mat, and sorrel, the seedling roots of which seem to have the power of boring through a fibrous layer of this sort. It is possible to remove the mat entirely by bacterial action alone, if sufficient lime be added periodically to make the reaction neutral, but failing these repeated additions the mat persists. We shall confine ourselves to the normal case where earthworms bring the source of energy into the soil. Directly the energy is available, it begins to be utilised. Two laws govern the change. The first is well-known to biologists: it states that the total energy of the system remains constant and can neither be increased nor diminished except from outside; in other words, that energy can be neither created nor destroyed. The second law is less familiar: it is that energy once transformed to heat by one organism cannot be used again by another. It is not destroyed; it remains intact, but is useless to the organism. One cannot have an indefinite chain of organisms living on each other’s excretory products; there was a certain quantity of energy in the food eaten by the first, and no more than this quantity can be got out whether one organism obtains the whole or whether others share it.