502 



ON INCREASE IN SIZE 



Table 68 [cont.). 

 Environmental temperatures (cont.). 



[PT. Ill 



* Hertwig's and King's figures are interesting because Rana fusea and temporaria lay 

 early in spring when the water is often freezing: Rana palustris and esculenta later, and 

 Bufo lentiginosus later still. Giglio-Tos has emphasised the importance of this in ecology. 



degree of external heat is reached, the mechanism of development 

 will be irreversibly interfered with and the growth-rate will suddenly 

 drop. On the cold side the growth-rate will fall off steadily. It is 

 necessary to distinguish these two types of effect, for it is only the 

 latter that concerns us here. According to the kinetic theory, rise 

 in temperature leads inevitably to an increased vigour of molecular 

 movement, and, as is well known, the extent to which this happens 

 gives a measure of the extent to which the process under examination 

 is physical or chemical. For in the former case the amount of increase 

 in molecular motion will only exert a direct effect in speeding up the 

 process, but, if the possibility of a chemical combination is present 

 to complicate matters, the effect will be much greater. The older 

 workers believed on these grounds that it would be very simple to 

 determine whether the nature of any given "master reaction" in 

 living matter was physical or chemical, but maturer consideration 

 and extended experiments have shown that such determination is 



