THE PROGRESS OF LIFE 95 



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 ; while 

 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 migrations 

 in all these cases were probably due to seeking out new 

 supplies of food ; and it is this which made Ptero- 

 dactyles, birds, and bats take to flying. 



Early Vertebrates and the Use of Armour. Again, why 

 did so many early vertebrates the Ostracodermi, the 

 Dipnoi and the Labyririthodonts acquire such a strong 

 armour ? Surely not to defend themselves from their 

 enemies ; for the Eurypterygians were the only large 

 animals living at the time, and they do not appear to 

 have been very formidable, as they have no apparatus 

 for catching a moderately active prey. The principle 

 of " following the food supply," will, I think, give us 

 an answer. They may have fed upon the sea-weeds, 



