560 Comparative Morphology of Chordates 



affecting the carnivores; too prolific reproduction, overpopulation, 

 crowding, and famine, perhaps inducing preying of carnivores upon 

 one another — some combination of these circumstances must have up- 

 set the biologic balance and precipitated the almost total reptilian 

 disaster. 



In Mesozoic times the earth's surface was much warmer than now. 

 Presumably, ancient reptiles were poikilothermous. In "cold-blooded"' 

 animals metabolism is slow. Relatively little internal heat is produced. 

 The reptilian skin affords little protection against loss of heat. For 

 maintenance of an internal temperature adequate for metabolism, the 

 reptile is largely dependent on the external medium. Reptiles flourish 

 in a tropic climate. At present, the larger reptiles — crocodiles, pythons, 

 giant monitor lizards — occur only in the tropics and warmer temperate 

 zones. In cooler temperate latitudes reptiles are smaller and fewer. In 

 the polar regions there are none. 



This relation of size to temperature is a matter of geometry. In 

 similarly shaped bodies of varying size, surface varies with the square, 

 and volume with the cube, of linear dimension. The large reptile has 

 relatively much less surface than the small one. At the surface of the 

 animal, heat may be either absorbed or lost, depending on the relative 

 internal and external temperatures. If, after a cool night, a 6-inch 

 lizard and a 12-foot alligator are lying in the warm morning sunshine, 

 the lizard will become warmed through much sooner than the alligator. 

 It was the world's extensive areas of warm climate that made possible 

 the spectacular Age of Reptiles. If reptiles had not appeared until 

 the late Mesozoic, probably there would never have been an Age of 

 Reptiles. 



The lower temperatures of the Cenozoic restricted to the earth's 

 warmer zones such large reptiles as had survived the late Cretaceous 

 disaster. Rut, while the gigantic and masterful Mesozoic reptiles were 

 passing out, the birds, only recently (in geologic time) arrived, not only 

 survived but increased, and in the course of time adapted themselves 

 to diverse environments. This they were able to do because they had 

 become homothermous. If that far-reaching occupation of the earth's 

 land surfaces which had been attained by reptiles was to be retained 

 by vertebrates during a cooling age, the two essential requirements 

 were warm-bloodedness and a method of reproduction which would 

 obviate exposure of the developing young to the hazards of environ- 

 mental temperatures. These requirements were, in fact, met by birds 

 (for it seems a safe assumption that high temperature and feathers were 

 acquired together), and from early avian times down to the present 

 there have been some birds which, incapable of flight, have lived as 

 biped land animals. Rut the entire structure of birds had become so 



