184 THE MEANING OF EVOLUTION 



mightily in favor of the mammals. Their reptilian 

 ancestors were cold blooded. When the climate was 

 warm they were active; when the climate was cold 

 they were sluggish. With the continuation of the 

 annual alternations of cold and warm weather that 

 had now set in upon the earth, the little birds and 

 mammals had in their warm blood an advantage 

 which, in the long run, enables them not simply to 

 compete with their reptile forefathers, but to outdis- 

 tance them absolutely in the race. Here and there, 

 on earth to-day, exist a few big reptiles like the croco- 

 diles and the boa constrictors. But they are few and 

 comparatively insignificant among the multitudinous 

 population of the globe and are confined to the hotter 

 portions of the earth. For the most part, the reptiles 

 now play an insignificant and unobtrusive part. The 

 little molelike creatures, practically unnoticed between 

 their feet in the later Mesozoic, have come to supplant 

 them entirely, and almost to rival them in size. While 

 the reptiles have grown steadily smaller, the mam- 

 mals have steadily become larger. 



While there is no land mammal to-day as big as 

 the heaviest of the reptiles in the Mesozoic, the whale, 

 which is one of the mammals that has again taken to 

 the ocean, surpasses in size even those gigantic crea- 

 tures. There never lived in the world before a 

 creature quite so big as the biggest of our whales. 



