THE PHYSIOLOGICAL EVOLUTION, ETC. 379 



any means equally affected in the different animals by 

 equal variations of temperature. In the following table are 

 given what were called the "temperature increments" of 

 the animals experimented on, or the relations of the re- 

 spiratory activity at 24 to the respiratory activity at io° : — 



Here it will be seen that the increments vary from 5*1 

 to 1 "9 ; or, if the various animals be grouped more or less 

 according to their morphological relationships, and means 

 taken, the numbers vary from 4/8 for the Ctenophores 

 examined, to 2*3, or less than half the amount, for the 

 teleost fish. With evolution of morphological structure, 

 there would, in fact, appear to be a gradual evolution of 

 increased power of resistance to variations of temperature. 

 The tunicate Salpa: appear to form an exception, but, 

 omitting them, we find that the temperature increment 

 gradually diminishes as we pass first to the two medusae 

 examined, then to the three mollusca, then to the acraniate 

 Amphioxus, and lastly to the teleost fish. As may be 

 gathered from the last column of the table, this gradual 

 evolution is accompanied by a gradual increase in the 

 percentage of solids in the tissues of the organisms. In- 

 deed, it seems highly probable that it is to this cause, 

 rather than to the increased complexity of morphological 

 structure, and growth of nervous control over the tissue 

 metabolism, that the increased resistance to temperature 

 changes is chiefly due. Thus all the animals in the above 



