222 ANIMALS AT WORK AND PLA Y 



habit the deep left light, warmth, and food, and 

 invaded the realms of cold and darkness, where their 

 eyes lost the power of sight, while retaining the 

 outward form which they possessed for use in the 

 sunlight. But Mr Battye has an alternative theory 

 to take the place of that which sees in the existence 

 of the Arctic mammal the relics of a northern 

 migration. He considers 'that the Arctic plants 

 and animals are there because they were born 

 there ; ' certain plants existed in the northern region 

 before they descended into temperate Europe ; and 

 there is ' no reason to suppose that contemporary 

 animal life could not have begun there also, when 

 we remember that these regions had then cooled 

 down from a temperature sufficiently tropical to have 

 supported the corals of the Silurian era which 

 formed the reefs now shown to have existed in those 

 regions.' Such a theory explains the northern 

 migration of birds by supposing that they are return- 

 ing to their old home ; and the musk-sheep, reindeer 

 and polar-bear remain there and refuse to leave for 

 the same reason. 



But it does not explain why the beasts, like the 

 birds, do not leave the country in the winter, and 

 settle in more southern latitudes, whence no tempta- 

 tion would induce them to return. 



