ENVIRONMENT, PAST AND PRESENT 47 



return again at maturity, maybe years later, but 

 the phenomenon is double and annual as applied 

 to the species. The individual returns but once 

 after the lapse of years, but every year there is a 

 procession of individuals. 



Such a definition is by no means an arbitrary one, 

 for one leg of typical migrations has reproduction 

 as its sole, or at all events, its most obvious goal. 

 Thus oceanic birds, which have never become wholly 

 emancipated from the land habits of their ancestors, 

 must return to land in order to breed, even though 

 they still have to depend on the ocean for their food. 

 Conversely most amphibia must breed in water 

 despite the fact that they spend the rest of their 

 lives on land. The land-crab of the West Indies, 

 Geocarcinas, comes down to the ocean to breed. 

 Its movements to the sea are annual and a return 

 inland follows upon breeding. Numerous sea- 

 snakes and marine turtles find themselves compelled 

 to come ashore to deposit eggs. Seals, sea-lions, 

 walruses and other aquatic mammals have never 

 solved the problem of giving birth to their young at 

 sea and, like any other mammals (except whales) 

 must be ashore to do so. On land they cannot feed 

 and reproduction is undoubtedly their driving force. 

 Various species of salmon and eels, some of which 

 undertake stupendous migrations, although spawn- 



