348 STRUCTURE OF THE VERTEBRATES 



tains. in process of formation accompanied by the regression of 

 the great inland sea which covered the Great Plains. The wide 

 marshes which bordered the sea were the home of many herbivo- 

 rous dinosaurs and their carnivorous enemies. The destruction 

 of the herbivores would cut off the food supply of the carnivores. 

 Botanical evidence indicates that the late Cretaceous was char- 

 acterized by a much cooler climate than had formerly existed, 

 and this may have been a contributing factor. In any event, 

 with the disappearance of the larger and more dangerous rep- 

 tiles, the conflict between the reptiles and the mammals became 

 less unequal and the better coordinated mammals won. 



E. From Reptile to Mammal 



Fossil remains give the story of the evolution of the mammals 

 from mammal-like reptiles. Unfortunately one is limited to the 

 skeletal remains in drawing conclusions from this group, for the 

 physiological changes were vastly more important, and here one 

 can depend only upon analogy with living groups. 



A higher basal metabolism is well correlated with the increase 

 of brain structure, and the calvaria of the ancestral reptile 

 group shows that their brains had kept pace with their other 

 mammalian characters. Equally good analogies are found in 

 the structure of the appendages and girdles. The typical reptile, 

 with large coracoids and his humerus and femur almost at right 

 angles to his body, moves rapidly for only a short time ; and the 

 reptile therefore gorges himself at one meal and lies down for 

 days or weeks of rest. But with a higher metabolic rate more 

 frequent eating is necessary, and this is correlated in all forms 

 with a running, active type of body. The Therapsid (mammal- 

 like) reptiles had a skeleton indicative of swift, prolonged mo- 

 tion. The body was slender, the coracoids were small, and the 

 elbows and knees were drawn in towards the vertebral axis of 

 the body. 



Further evidence is from analogy with the higher reptiles and 

 more primitive mammals. It is evident from living mammals 

 (armadillos and Manis) that hair and scales exist at the same 

 time. Hairs probably arose as sensory structures, and surely 

 have small function in an animal like Manis; but in the higher 



