No. 381.] A HALF-CENTURY OF EVOLUTION. 659 
lose sight of the fact that the changes in the earth’s population 
were due not less to biological than to geological and topo- 
graphical factors. The process of extinction was favored and 
hastened by the incoming of more specialized forms, many of 
them being carnivorous and destructive ; as, for example, nearly 
all fishes and reptiles live on other animals. The struggle for 
existence between those which became inadapted and useless 
in the new order of things went on more actively than at pres- 
ent. The process of extinction of the higher, more composite 
amphibians (the labyrinthodonts) was largely completed by the 
multitude of theromorphs and dinosaurs which overcame the 
colossal Cheirotherium, Mastodonsaurus, and their allies. 
During the centuries of the Trias the lowlands became 
crowded, and the reptilian life was forced in some cases to gain 
a livelihood from the sea, for at this time was effected the 
change from small terrestrial reptiles like Nothosaurus to the 
colossal plesiosaurs and ichthyosaurs, in which digitate limbs 
were converted into paddles; and the ocean, before this time 
uninhabited by animals larger than ammonites, cuttles, and 
sharks, began to swarm with colossal vertebrates, the increased 
volume of their new and untried habitat resulting in a tendency 
to a corresponding increase in weight, just as whales, which 
possibly evolved from some land carnivore in the early Tertiary, 
waxed great in bulk, the increase in size perhaps having been 
due to the great volume of their habitat, the ocean. 
Nothing so well illustrates the advantage to an incipient type 
as entering a previously uninhabited topographical area, or a 
new medium, such as the air, in the case of the pterodactyles, 
‘the first vertebrates to solve the problem of aérial flight. 
Originating and prospering in the early Mesozoic, they held 
1 After writing the above lines I find the same view expressed in Woodworth’s 
Base-Levelling and Organic Evolution. He remarks: “ The exact cause of thei ir 
(P. 225). Regarding the circumstances favorable to reptilian life, he also states : 
“In the development of the peneplain from the high relief of the Permian and 
again at the close of the Jur =i the widening out of the lowland, with plains 
favored the water-loving reptilia. It is to these eop circumstances, I 
think, that we must look for our explanation of the remarkable history of this 
class in Mesozoic times ” (p. 226). 
