232 THE RATE OF GROWTH [ch. 



The constancy of these results might tempt us to look on the 

 phenomenon as a simple one, though we well know it to be highly 

 complex. But we had better rest content to see, as Arrhenius saw 

 in the beginning, a general resemblance rather than an identity 

 between the temperature-coefficients in physico-chemical and 

 biological processes*. 



It was seen from the first that to extend Van't Hoff's law from physical 

 chemistry to physiology was a bold assumption, to all appearance largely 

 justified, but always subject to severe and cautious limitations. If it seemed 

 to simplify certain organic phenomena, further study soon shewed how far 

 from simple these phenomena were. Living matter is always heterogeneous, 

 and from one phase to another its reactions change; the temperature- 

 coefficient varies likewise, and indicates at the best a summation, or integration, 

 of phenomena. Nevertheless, attempts have been made to go a little further 

 towards a physical explanation of the physiological coefficient. Van't Hoff 

 suggested a viscosity-correction for the temperature -coefficient even of an 

 ordinary chemical reaction; the viscosity of protoplasm varies in a marked 

 degree, inversely with the temperature, and the viscosity-factor goes, perhaps, 

 a long way to account for the aberrations of the temperature-coefficient. It 

 has even been suggested (by Belehradekf) that the temperature- coefficients 

 of the biologist are merely those of protoplasmic viscosity. For instance, the 

 temperature-coefficients of mitotic cell-division have been shewn to alter 

 from one phase to another of the mitotic process, being much greater at the 

 start than at the end| ; and so, precisely, has it been shewn that protoplasmic 

 viscosity is high at the beginning and low at the end of the mitotic process §. 



On seasonal growth 



There is abundant evidence in certain fishes, such as plaice and 

 haddock, that the ascending curve of growth is subject to seasonal 

 fluctuations or interruptions, the rate during the winter months 

 bejng always slower than in the months of summer. Thus the 

 Newfoundland cod have their maximum growth-rate in June, and 

 in January-February they cease to grow; it is as though we super- 

 imposed a periodic annual sine-curve upon the continuous curve of 

 growth. Furthermore, as growth itself grows less and less from 

 year to year, so will the difference between the summer and the 



♦ Cf. L. V. Heilbronn, Science, lxii, p. 268, 1925. 

 t J. Belehradek, in Biol. Reviews, v, pp. 30-58, 1930. 



X Cf. E. Faure-Fremiet, La cinetique du developpement, 1925; also B. Ephrussi, 

 C.R. cLxxxii, p. 810, 1926. 



§ See {int. al.) L. V. Heilbronn, The Colloid Chemistry of Protoplasm, 1928. 



