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. 
