[41] _E. Hering_: “Zur Theorie der Vorgänge in der lebendigen Substanz.” In Lotos, Bd. 9, Prag. 1888. [42] _Max Verworn_: “Allgemeine Physiologie. Ein Grundriss der Lehre vom Leben.” V. Aufl. Jena 1909. In considering the nature of the equilibrium of metabolism one factor has been disregarded which must be taken into account at every point; this is growth. Growth changes, although varying more or less, are never absent during the life of the organism. An equilibrium of metabolism never exists in a strictly mathematical sense, and here again we are working with a conception which is faulty, because it is an abstraction, originating from experience with rather too restricted boundaries. But an error of which one is aware is not dangerous. In mathematics we also consciously reckon with errors, without the result being altered. In the before mentioned cases the equilibrium of metabolism was maintained, because the investigations involved only a short time in an adult mammal. In the adult mammal the growth processes occur very slowly, so that alterations within a relatively short time are not demonstrated. If it were possible to subject the adult mammal to metabolic or calorimetric experiments, extending for years, it would be found that the intake would be qualitatively and quantitatively different at the end of the investigation and that the same would apply to the outgo. In the growing egg cell this takes place with much more rapidity. In the organism which rapidly grows, it can be seen at once that the quantity of the outgo of the products of disintegration cannot be equal to that of the intake of foodstuffs. If biotonus were equal to one, the organism could not grow. Equilibrium of metabolism can only be understood when we take into consideration a period of time in which the alterations in growth take place with such imperceptible slowness that the resultant error is inconsiderably minute. This period of time is of greatly varying length in different living organisms and this fact must be taken into account in every living form. Only with this restriction can we justify the use of the term “equilibrium of metabolism.” Then, however, its use is of great value. The _metabolism of stimulation_ is then a disturbance of the metabolism of rest, that is, a disturbance of the equilibrium of metabolism through the effect of stimuli.