That is almost all[107] that is actually known of adaptation with regard to the use of an abnormal food supply. Though important, it cannot be said to be very much. But could we expect very numerous regulations here at all after what we laid down in a former paragraph about the possibilities of adaptive regulation in general? The functional state must have been altered in order that such regulations may occur. Now there is no doubt that this state may be really altered only if an abnormal food has first been taken in altogether by the cell-protoplasm of the body-surfaces, but never if it has only entered the cavity of the intestine, which, strictly speaking, is a part of the exterior medium. Fungi indeed not only take in the abnormal food, but also know what to do with it, but all animals are obliged to treat first with their chemical secretions what happens to be present in their intestine, in order that it may be taken up by their living cells, and one hardly can wonder that these secretions are only formed in correspondence to a limited number of outside stimuli. In fact, as soon as we look upon what adaptive or regulatory work happens in metabolism *inside* the body interior, we meet, even in animals, regulations of a far more developed type. [107] The discovery of Weinland that adult dogs are able to produce “lactase” in their pancreas, whenever they are fed, quite abnormally, with milk-sugar, has recently been said to be vitiated by an analytical mistake. Discoveries of the last few years have taught us that almost all metabolic processes in the organism, including oxidation, are carried out by the aid of special materials, the so-called enzymes or ferments. These are known to exist in the most different forms even in the inorganic world. They are simply chemical compounds, of specific types, that bring about chemical reactions between two other chemical materials, which in their absence would either not go on at all or would go on very slowly. We cannot enter here into the much disputed chemical theory of what is called “catalysis”: we can only say that there is no objection to our regarding almost all metabolic processes inside the organism as due to the intervention of ferments or catalytic materials, and that the only difference between inorganic and organic ferments is the very complicated character of the latter and the very high degree of their specification.