ACCLIMATION IN PLANARIA DOROTOCEPHALA. 311 



regulating mechanism such as that of warm-blooded animals 

 seem to be that the former acts relatively slowly and only par- 

 tially compensates the direct effect of temperature change; 

 whereas the temperature-regulating mechanism acts very rapidly 

 and maintains a uniform body temperature over a wide range of 

 external temperature change. 



Since the establishment of the fact that the usual temperature 

 coefficient of velocity of chemical reaction for 10 C. within ordi- 

 nary temperature range is between 2 and 3, and that the tempera- 

 ture-coefficients of many physical processes are of very different 

 orders of magnitude, the temperature coefficients of various 

 physiological processes have been determined on the assumption 

 'that such coefficients will show whether chemical or physical 

 processes are primarily concerned. Snyder ('08) found that the 

 velocities of nerve impulses follow van't Hoff's law for chemical 

 reactions. Van Slyke and Cullen ('14) observed that the re- 

 action rate of the enzyme urease is nearly doubled by every 10 

 rise in temperature between 10 and 40, the average temperature 

 coefficient being 1.91 within this range. Loeb and Wasteneys 

 ('n) investigated the temperature coefficient for the rate of 

 oxidation in newly fertilized Arbacia eggs and found it to be 

 remarkably constant, about 2. And Lillie and Knowlton ('97) 

 showed the temperature coefficient for the development of the 

 egg of the frog to be of the same nature as that for chemical 

 reactions. A great number of other similar cases might be 

 quoted. Yet with the increase in knowledge along this line it 

 has become increasingly evident that caution is necessary in 

 generalizing from such data ; for not only are some of the chemical 

 and physical temperature coefficients almost identical, but as 

 Bayliss ('15, p. 43) and others have pointed out, the physio- 

 logical processes are processes occurring in heterogenous systems 

 in which even when a chemical reaction occurs, other processes 

 must also be concerned; and the velocity of the whole will be 

 conditioned by that process the rate of which is lowest under the 

 given conditions. 



The experiments on temperature acclimation in Planaria show 

 that in addition to the direct effect of temperature on the velocity 

 of cer ain physiological processes, there is a secondary effect 



