90 THE FITNESS OF THE ENVIRONMENT . 



the first place marked influence of change of 

 temperature upon chemical reaction is almost 

 universal, and as a rule an increase of 10° 

 centigrade in temperature will more than 

 double the rate of a chemical change. 1 Sec- 

 ondly all living organisms contain both chemi- 

 cal substances and physico-chemical struc- 

 tures or systems which begin to be altered, 

 and usually irreversibly altered, at a tempera- 

 ture which is very little above that of the 

 human body. 2 It is perhaps imaginable that 



1 If the velocity of a chemical reaction be represented by 

 a coefficient, k, the increase in its magnitude with rising tem- 

 perature is unlike that of ordinary physical coefficients, and 

 in many cases amounts to a two or threefold rise for a tem- 

 perature increase of 10° centigrade. The well-known data 

 concerning the transformation of dibromsuccinnic acid into 

 brommaleic acid and hydrobromic acid in aqueous solution 

 illustrate a typical case. 



2 This is attested not only by the low temperature at 

 which many proteins coagulate, but also by the action of 

 temperatures between 50° and 60° to inactivate enzymes, and 



