THE PKOGRESS OF LIFE 97 



sharks, which began to be common in the Cretaceous, 

 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 sometimes 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 croco- 

 diles survived, by learning to bury their eggs in sand 

 or mud. This, however, will not account for the destruc- 

 tion of the Ichthyosaurians, 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 

 difficult to apply, on account of our ignorance. 



One thing is evident. There is no general law, 

 either for development or for extinction, and, con- 

 sequently, there can be no general organic principle 

 regulating the length of duration of groups, as there is 

 with individuals. An analogy between the two cases 

 may sometimes be traced, but it is very incomplete, and 

 becomes misleading if treated as more than an analogy. 



Nevertheless, Professor Cope has put forward an 

 hypothesis, which he calls " Expression Points," l 



1 "Origin of the Fittest," p. 25 (1892), and ''Primary Factors of 

 Organic Evolution," p. 25 (1898). 



G 



