110 THE RATE OF GROWTH [ch. 



Van't Hoffs law, which has become a fundamental principle 

 of chemical mechanics, is likewise applicable (with certain qualifica- 

 tions) to the phenomena of vital chemistry ; and it follows that, 

 on very much the same lines, we may speak of the "temperature 

 coefficient" of growth. At the same time we must remember 

 that there is a very important difference (though we can scarcely 

 call it a fundamental one) between the purely physical and the 

 physiological phenomenon, in that in the former we study (or 

 seek and profess to study) one thing at a time, while in the latter 

 we have always to do with various factors which intersect and 

 interfere ; increase in the one case (or change of any kind) tends 

 to be continuous, in the other case it tends to be brought to arrest. 

 This is the simple meaning of that Law of Optimmn, laid down by 

 Errera and by Sachs as a general principle of physiology : namely 

 that every physiological process which varies (like growth itself) 

 with the amount or intensity of some external influence, does so 

 according to a law in which progressive increase is followed by 

 progressive decrease ; in other words the function has its optimum 

 condition, and its curve shews a definite maximum^. In the case 

 of temperature, as Jost puts it, it has on the one hand its accelerat- 

 ing effect which tends to follow van't Hoff's law. But it has also 

 another and a cumulative eft'ect upon the organism : " Sie schadigt 

 oder sie ermiidet ihn, und je hoher sie steigt, desto rascher macht 

 sie die Schadigung geltend und desto schneller schreitet sie voran." 

 It would seem to be this double effect of temperature in the case 

 of the organism which gives us our "optimum" curves, which are 

 the expression, accordingly, not of a primary phenomenon, but 

 of a more or less complex resultant. Moreover, as Blackman and 

 others have pointed out, our '"optimum" temperature is very 

 ill-defined until we take account also of the duration of our experi- 

 ment ; for obviously, a high temperature may lead to a short, 

 but exhausting, spell of rapid growth, while the slower rate 

 manifested at a lower temperature may be the best in the end. 



Quantitative Relation between Temperature and Standard Metabolism, l7it. 

 Zeifschr. J. physik.-chem. Biologie, i, p. 491, 1914; Piitter, A., Ueber Temperatur- 

 koefficienten, Zeiischr. f. allgern. Phi/siol. xvi, p. 574, 1914. Also Cohen. 

 Physical Chemistry for Physicians and Biologists (English edition), 1903; Pike, 

 F. H., and Scott, E. L., The Regulation of the Physico-chemical Condition of the 

 Orsranism, American Naturalist, Jan. 1915, and various papers quoted therein. 



