In another direction likewise heat depression is of special interest, that is, in regard to the theory of nature of the processes in the living substance. According to the _van’t Hoff_ law we may assume that every individual constituent metabolic process, if we imagine it as isolated and taking place in a test tube, undergoes in more or less the same degree as all others an increased rapidity of reaction as a result of increased temperature. At the same time, in living substance we find on the contrary that the _van’t Hoff_ law is only within certain narrow limits more or less applicable to the sum total of all metabolic processes. Beyond certain degrees of temperature no further increase of the vital process takes place, instead a retardation occurs. The analysis of depression produced by heat shows us in the clearest and simplest manner the reason for this apparent deviation from the general law of _van’t Hoff_. This reasoning is based on the fact that the rapidity of reaction of a chemical process is not merely dependent upon the temperature, but likewise upon the mass relations of the reacting substances. In spite of the effect of the temperature in increasing the rapidity of reactions, the process undergoes retardation which extends to a complete cessation if the supply of material necessary to its existence does not keep pace with the increase produced by temperature. In the present instance the amount of reserve supplies for the building up of the disintegrating molecules exists in abundance, and it is merely the available oxygen which is in relatively a very small quantity. As soon, however, as metabolism in its entirety, or even merely in those parts in which oxygen is directly required, is increased by whatever means, the oxydative processes would be the first to fail and it must be from this point that the disturbance of the harmony in the interacting of the individual metabolic processes proceeds. This principle which we here see manifested in its simplest form in the effect of temperature on oxygen exchange in the form of a disturbance in the correlations of the individual constituent processes based on an alteration of the mass relation and the rapidity of reactions of individual members is, however, not merely restricted to effects of temperature and the results quickly following on a relative oxygen deficiency. It has, indeed, a much more general significance for all manner of constituent metabolic processes, for it is applicable to all nutrition and to all growth, and forms one of the most important factors which influence the process of development, that is, the gradual “metachronic” alterations in metabolism to which all living systems are subjected as long as life endures.