MIGRATION OF BIRDS. 501 



suffer this fate ; others it puts to sleep, they " hibernate ; " 

 others escape from the unfavourable conditions by wandering 

 away to warmer lands. The indirect effect is through the 

 lessening of the food supplies ; this is probably in most cases 

 the most potent cause of migration. It is natural that birds, 

 gifted above most creatures with powers of long and rapid 

 locomotion, should be the most noted travellers. 



As already stated nearly all birds migrate to some extent, 

 but the length of the journey is very various. Some birds of 

 tropical countries are said to move from the hills to the 

 valleys and back again ; those of regions of more variable 

 climate may perform flights of hundreds of miles, at times 

 even over the sea. There are fairly well marked paths of 

 migration, such as from Russia, down the west coast of 

 Norway, across the North Sea to Britain ; another from 

 Spitzbergen along the same course, but continued, through 

 France and Spain, to the west coast of Africa, and so on. 

 River valleys form minor routes. 



The thing that is so difficult to understand about these 

 long migrations is, how do the birds find their way, not only 

 over land, but over the trackless sea, and how do they 

 return with such precision to the same spots year after year ? 

 Perhaps in the case of those birds that fly in flocks the older 

 one^, who have performed the journey before, lead the 

 others ; even then the thing is wonderful. But many birds do 

 not fly in flocks ; and further, migrations are often performed 

 in the darkness of night. 



Wallace's theory of the origin of the habit and its per- 

 fecting by aid of natural selection is now widely accepted. 

 He supposes in past times a more equable climate than at 

 present ; as the seasons become more distinct, the birds had 

 to fly further and further ; those that did not fly at the right 

 time and in the right direction would, as the necessity for 

 migration became more imperative, become extinct, and so 

 the habit might become fixed as an instinct. 



Intelligence and Emotions. — As birds usually live a full 

 life, with variety of function and stimulating environment, 

 it is natural that their sense-organs and nervous system should 

 be highly developed, that they should be clever. More- 

 over, their parental care and their courtships, while of course 

 presupposing emotion, must have tended to foster it. While 



