We now turn to study the cases of a compensation of nourishments serving for the real building up of the organism. Albumen, we know, is absolutely indispensable for animals, even for adults, though nothing is known about the purpose it serves in the latter; its place can be taken of course by those less complicated compounds which result from its first decomposition, effected by pepsin and trypsin, but nothing else will do. The salts of sea-water, according to Herbst’s experiments, may only vary to a very small degree if the development of marine animals is to go on well; potassium may be replaced by caesium or rubidium, and that is all. Much the same is true of the salts necessary to plants. It will not surprise us very much to hear that algae can also be successfully fed with the potassium salts of organic compounds, and higher plants with acid amides or glucoses instead of carbonic acid, as those products are normal steps in their assimilation; and it may also be fairly easily understood that nitrogen can be offered in organic form instead of as a nitrate. It was in the group of fungi that really important adaptations with regard to the proper form-producing alimentation were first discovered, and these are of a very complicated kind indeed. Fungi are known to be satisfied with one single organic compound instead of the group of three--fat, carbohydrate and albumen--necessary for animals. Now Pfeffer showed that the most different and indeed very abnormal compounds were able to bring his subjects to a perfect growth and morphogenesis; and, moreover, he found that, if several kinds of such food were offered together, they were consumed quite indifferently as to their chemical constitution, but only with regard to their nutritive value: that sort of food which had produced a better growth than another when both were offered separately was found to save the latter from consumption whenever both were offered together. Here we are faced by one of the most typical cases of regulations in metabolic physiology: the organism is able to decompose compounds of the most different constitution, which have never been offered to it before; but nevertheless, it must remain an open question whether real “secondary” regulation has occurred, as nothing is known in detail about the single steps of metabolism in these fungi. There *might* be some ferments equally able to destroy different classes of compounds,[106] and that the most nutritive compound is used up first *may* be a question of physico-chemical equilibrium. [106] In all cases where fungi of the same species are able to live on different hosts, that is, to penetrate membranes of a different chemical character, a similar objection as to the “secondary” type of such a regulation may be made.