PALAEONTOLOGY 79 



most remarkable thing in the development of the 

 land vertebrates is the number of times in which 

 they returned to the sea. Vertebrates, commencing 

 .as marine animals, have, like plants, only once 

 .achieved the task of becoming thoroughly adapted for 

 terrestrial life ; while land vertebrates have taken to 

 living in the ocean many times. In the upper Trias 

 we find Ichthyosaurus and Plesiosaurus ; in the 

 Jurassic, the turtles ; in the Cretaceous, the Pythono- 

 morpha ; in the Eocene, snakes and sea-cows ; in the 

 Oligocene, penguins and whales ; and in the Pliocene, 

 seals. Each of these made an independent entrance 

 into the ocean. The reason for this difference is 

 probably due to the difficulty of water-breathing 

 vertebrates becoming air-breathers, while air- 

 breathers can more easily live in the sea. This, 

 however, was not the case among the invertebrates ; 

 for, in addition to insects and arachnida, several 

 different groups of Crustacea, mollusca, and worms 

 have attained a footing on the land ; w y hile only a 

 few of the land shells have gone back to an aquatic 

 life. Perhaps more interesting than all is the fact 

 that in the Miocene some of the turtles returned to 

 the land and became tortoises. 



But why did these migrations take place at all? 

 Were they to escape from enemies or were they to 

 obtain food? In the case of the large reptiles we 

 cannot suppose that they took shelter in the sea from 

 their foes on land. It is far more probable that they 

 began by trying to catch the fish which had so 

 largely increased in the Mesozoic sea ; and the same 

 may be said for the whales, penguins, and seals. The 



