94 THE LESSON OF EVOLUTION 



unbridged ; and palaeontologists are undecided as to 

 whether the former are descended from the Dipnoi or 

 from the Crossopterygii ; for the origin of the five-toed 

 limb in the amphibians is obscure. Also we feel no 

 certainty about the course of development among the 

 fishes themselves, and about the line of descent of the 

 lowest fishes from invertebrates. 



The reason for this is obvious. From the am- 

 phibians upwards we have animals with a hard 

 skeleton, easily preserved ; while from the amphibians 

 downward the skeleton is cartilaginous. Our clue is 

 lost, and our knowledge is almost entirely confined to 

 those exceptional animals which developed a hard 

 dermal armour. The whole of the facts may be 

 summed up in this sentence : The more complete our 

 knowledge of extinct animals, the clearer is the evidence 

 for development. This really amounts to a proof of 

 the theory, which was originally arrived at by biologists 

 from a study of living animals, and is now confirmed 

 by palaeontologists through a study of the animals 

 which formerly inhabited the earth. 



Migrations from the Land to the Sea. Perhaps the 

 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 Pythonomorpha ; in 

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

 penguins and whales ; and in the Pliocene, seals. 





