FITNESS OF TEMPERATURE CONSTANCY 95 



it was used as an indication of ^^ vitalism.'' It is now 

 known that with enzyme action, as with certain other 

 chemical reactions, a rise in temperature quickens cataly- 

 sis, in accordance with the temperature coefficient of 

 chemical processes generally, but also renders an enzyme 

 less stable, so that it breaks down more and more rapidly 

 as the temperature rises. The temperature at which the 

 maximum rate is shown is therefore that at which the 

 acceleration due to temperature is in greatest excess over 

 the simultaneous destruction of the enzyme, and is known 

 as the optimum. Thus the fitness of the constant tem- 

 perature level of homoiotherms can be readily appre- 

 ciated. 



Heat is not to be regarded as the cause of body proc- 

 esses; it is only one of the conditions. The extent to 

 which heat may influence such processes is relative and 

 varies with the quantity available in kinetic form. The 

 constancy of temperature maintained at the particular 

 level represented in the homoiotherms permits the con- 

 tinuity of the physiological processes of animals at their 

 maximum effectiveness, independent of environmental 

 caprice — and therein lies its fitness! 



