82 THE LESSON OF EVOLUTION 



difficult to explain. This rapid extinction seems to 

 have occurred nearly simultaneously all over the 

 world in Europe, in North America, in Australia, 

 and in New Zealand. In none of these countries 

 do we find the Cretaceous reptiles and cephalopods 

 living with the Eocene birds and mammals ; although 

 such is said to have been the case in Patagonia. 



We can only speculate on the causes. Perhaps 

 the cephalopods were killed off by the predaceous 

 sharks, which began to be common in the Creta- 

 ceous, and were very abundant in the Eocene. Dr. 

 C. A. White has pointed out that in North America 

 the climate and other physical conditions of the 

 Cretaceous period were continued into the Eocene ; 

 so that the Cretaceous dinosaurians could not have 

 been killed off by a change in climate, but probably 

 succumbed to the Eocene mammalia in the unequal 

 struggle for existence. But if so, we ought some- 

 times to find their remains commingled ; and we 

 know that the marine reptiles could not have been 

 killed off by the marine mammalia, for the former 

 died out before the whales came on the scene. It 

 seems to me more probable that the rapid extinction 

 of the Mesozoic reptiles was due to the destruction 

 of their eggs by the early birds or mammals ; and that 

 the turtles and crocodiles survived, by learning to 

 bury their eggs in sand or mud. This, however, will 

 not account for the destruction of the Ichthyo- 

 saurians, which were viviparous. 



Probably we have in the principle of natural 

 selection, acting through the food-supply, a solution 

 of all these varying phases ; but it is a principle 



