16 Introduction 



chaetes, polychaetes, onychophorans, myriapods, crustaceans, insects, 

 arachnids, gastropods, and vertebrates are well established as land 

 animals. Some of these are still obliged to live in moist situations, 

 but such well-adapted types as certain snails, arachnids, insects, 

 reptiles, birds, and mammals can exist even in deserts. Certain of 

 the animals that have attained land life are of primitive marine 

 stocks (Harms, 1929), but most successful land animals are mod- 

 ern, progressive types with impervious integuments and mechanisms 

 for conserving water while breathing air. 



In the four succeeding chapters the emigrations of animals from 

 sea to land will be considered in some detail: the routes the emi- 

 grants followed; the reasons why animals left the dependable, stable 

 ocean for a precarious life on land; the changes that have taken 

 place in structures and functions of the animals that have succeeded 

 in adjusting their systems of activities to land life; and the rewards 

 that accrue to those animals that have struggled up long difficult 

 trails and now view the world from mountain peaks. This has been 

 the greatest emigration the world has known. 



