AND ITS INHABITANTS 127 



Origin of warm blood. Toward the close of the Pennsyl- 

 vanian commenced that series of earth movements which was 

 to culminate in the Appalachian revolution and the passing of 

 the Paleozoic, and as attendant phenomena came increasing 

 aridity and, early in Permian time, glaciation, which, in the 

 southern hemisphere, exceeded in intensity and extent the 

 so-called glacial period of the Pleistocene. 



Aridity has its influence upon terrestrial types, at any rate 

 upon the more progressive of them, in that it places a premium 

 upon traveling powers, especially upon speed, for not only 

 are food and water scarce and far between, but the strife 

 between the pursuer and pursued becomes intensified neither 

 can afford to be outdistanced by the other. This means in- 

 creased metabolism, which in turn generally implies not only 

 greater motive powers but higher temperature. With in- 

 creasing cold a premium would be placed upon such creatures 

 as could maintain their activity beyond the limits of the shorten- 

 ing summers, and this could only be accomplished by the de- 

 velopment of some mechanism whereby a relatively constant 

 temperature could be maintained within the animal regardless 

 of external conditions; in other words, warm, as opposed to 

 cold (really variably temperatured or poikilothermous) blood. 

 This crisis means much, for the cold-blooded reptile has its 

 most decided limitations. On the other hand, the evolution of 

 the bird and mammal, the latter particularly, was rendered 

 possible by the concurrence of these two factors, aridity and 

 cold. Thus while actual recorded mammals (Upper Triassic) 

 and birds (Upper Jurassic) are younger in time, their incep- 

 tion could hardly have been later than the Permian. Not that 

 warm blood was at once attained, for that we believe to have 

 been a relatively slow process, just as was the emergence. 

 Indeed, in the existing egg-laying monotreme mammals the 

 blood still ranges in temperature through at least 30 F., so 

 with them the mechanism is not yet perfected. 



