66 THE ORIGIN OF MAN 



small grasping front limbs have been trans- 

 formed into wings. This evolution probably took 

 place earlier than the Upper Jurassic where rep- 

 tilian birds are known. The dinosaurs ances- 

 trally were doubtless water-loving quadrupeds, 

 but the increasing aridity of the climate com- 

 pelled them to travel rapidly and far in the search 

 of food and drink. And this was probably the 

 impelling force that raised these creatures from 

 the prone gait and posture and stimulated their 

 rapid evolution into the several types. As to ex- 

 tinction of this mighty race there are numerous 

 conjectures. One is that a severe change of cli- 

 mate with a diminution of the necessary heat 

 without which no reptilian race may thrive, or of 

 the moisture with an accompanying change of 

 vegetation was the cause. Another the inter- 

 necine warfare amongst the dinosaurs themselves 

 and another of the destructive slaughter, not of 

 adults but of the young, possibly while yet in the 

 egg, by some small bloodthirsty mammals. But 



