THE MAMMALS 135 



ency to maintain body temperature and physical activity 

 in cool weather must early have become an important 

 factor in mammalian evolution and, if skulls and teeth 

 are any criterion of mammalness/ selection may have 

 been working in this direction even among the Permian 

 Therapsida. 



The Permian was characterized by climates more 

 rigorous and sharply zoned than had prevailed since the 

 pre-Cambrian, with widespread glaciation at altitudes 

 and a marked drop in temperature in all latitudes. It 

 was also a period of aridity, in which the great coal- 

 making swamps of the Pennsylvanian contracted to 

 isolated locations, to be replaced in large part by desert 

 sand. So severe were the conditions of life that many 

 types of animals, both on land and sea, went down to 

 extinction, and others were greatly reduced in varie- 

 ties and numbers— for which reason the paleontologist 

 speaks of the 'Permian stricture' as though it were a 

 needle's eye through which the rich and diversified 

 fauna of the Pennsylvanian could not pass. 



It is tempting to see the pressiu-es of selection, in the 

 form of low temperatures prevailing over so large a part 

 of the world, and widespread aridity, working more or 

 less independently on the derivatives of the cold-blooded 

 amniotes which, in the Pennsylvanian, had already par- 

 tially adapted to aridity through the evolution of the 

 amniotic egg. (See Figure 9. ) In the Permian the reptiles 

 went one step further in adaptation to aridity by the 

 evolution of the uric acid habitus, and it was possibly 

 not until the late Triassic or early Jurassic that a bipedal 

 stem of these uric acid-excreting reptiles adapted to 

 frigidity, to give rise to the warm-blooded birds. There 

 is nothing, however, to argue against the view that the 

 pro-avian reptilian stem may have acquired some con- 

 trol of body temperature as far back as the Permian- 

 no reptile that survives today can throw any light on the 

 matter, the nearest relatives to the birds being the croco- 

 diles and alligators. 



