170 THE FITNESS OF THE ENVIRONMENT 



vation also provides a striking example of 

 the efficiency with which the reaction of sea 

 water is maintained. In spite of the unusual 

 circumstances here the variation is inconsider- 

 able. The only known important factor which 

 operates to establish and to preserve the re- 

 action of sea water is the carbonate equilib- 

 rium. 



There is one consideration which must be 

 especially noted before passing on. The most 

 obvious effect of slight changes of temperature, 

 and of slight changes of alkalinity as well, 

 is upon the velocity of chemical reactions. 

 In this respect the effect of hydroxyl ions is 

 likely to be in proportion to their concentra- 

 tion, 1 and the effect of temperature is usually 

 such that a change of about ten degrees doubles 

 the velocity of the reaction. 2 Hence ordinary 

 chemical reactions will progress about eight 

 times as fast in the hottest as in the coldest 

 ocean waters, and about seven times as fast 

 in the most alkaline as in the least alkaline 

 parts of the ocean. But in the case of any 

 organism inhabiting a particular locality such 

 changes in reaction velocity will be scarcely 



1 The chief actions of hydroxyl ions are catalytic, and, as 

 in the case of the catalysis of esterification, the effect is pro- 

 portional to the concentration of the hydroxyl ions. 



2 This observation is due to van't Hoff. 



