Glass Mammalia 



15 



Origin — General Characteristics 



Lungs and pentadactyl limbs make possible for most amphibians 

 terrestrial life of a very restricted sort. But aside from lungs and legs — 

 and some fishes have lungs — amphibians are ichthyopsidan. Their 

 delicate mucous skin compels them to live in moist places. Their 

 aquatic larval stage forbids them to wander far from ponds and 

 streams. Reptiles, replacing external mucus by the dry, impervious 

 protective stratum corneum, are able to withstand dry air. The rep- 

 tilian egg, deposited on land, develops an equipment of membranes 

 which serve for the protection, nourishment, and respiration of the 

 young animal, thus enabling it to attain an advanced stage of develop- 

 ment and growth before hatching. Therefore reptiles, relieved of all 

 necessity for living near bodies of water, were able to disperse them- 

 selves widely and become dominant terrestrial animals. Secondarily, 

 however, some of them reverted to aquatic life and some acquired the 

 power of flight. Birds, sauropsidan in a general way, elaborated the 

 stratum corneum into feathers, transformed the pectoral limb into a 

 wing, and stabilized internal temperature at a high level, thus achiev- 

 ing that perfection of aerial locomotion which opened to them new 

 realms of living. 



With fishes perfectly adapted to aquatic life, reptiles capable of 

 acquiring mastery of the earth's land surfaces, and the air well popu- 

 lated by birds, it would seem that the vertebrates had fulfilled all 

 possibilities. But something went wrong with reptiles. In late Mesozoic 

 times they suffered a rapid decline, and only a feeble remnant survives 

 to the present. Geologic evidence gives no fully satisfying explanation 

 of the cause of the decline. Changes in configuration of land surfaces; 

 falling temperature; reduced atmospheric moisture; nature of vegeta- 

 tion altered to the disadvantage of the herbivores and thus indirectly 



559 



