THE PHYSIOLOGICAL EVOLUTION OF THE 

 WARM-BLOODED ANIMAL. 



IN physiological text-books it is customary to point out 

 how widely warm- and cold-blooded animals differ from 

 each other in the absolute temperature of their bodies, and in 

 the manner of their reaction to change of external tempera- 

 ture. In practically no case, however, is an attempt made to 

 connect the two classes of animals, and to show, by means 

 of connecting links, how the one class may have been 

 evolved from the other. Doubtless this was in part due to 

 actual lack of data. But now, in consequence of the recent 

 publication of a very interesting paper by Sutherland, 1 

 on the temperatures of monotremes and marsupials, it is 

 possible to trace with more or less completeness the various 

 stages by means of which the lower invertebrates may have 

 been gradually evolved, in a physiological sense, to produce 

 a warm-blooded animal, such as man, in which the nervous 

 system appears to possess almost perfect power of keep- 

 ing the temperature of the body constant, whatever be the 

 temperature, or variations in the temperature, of the im- 

 mediate environment. 



It might be thought that, in their reaction to tempera- 

 ture, all cold-blooded animals are alike. But this is by no 

 means the case. There has taken place among them a 

 gradual evolution of the nervous control of the tissue meta- 

 bolism, which is probably quite as great as that separating 

 the typical warm-blooded animal from the higher cold- 

 blooded one. There would even seem to be a gradual 

 evolution in the reaction of tissue change to temperature 

 in respect of the tissues themselves, apart from a special 

 nervous controlling influence. Thus the writer," as the 

 result of observations on the respiratory activity of various 

 marine invertebrate and vertebrate animals, at various 

 temperatures, came to the conclusion that this was not by 



1 Proc. Roy. Soc. Victoria, vol. ix., p. 57, 1897. 



2 J. Physiol., xix., p. 18, 1896. 



