7 8 IN BIRD LAND. 



on account of the exigencies of food, to scatter 

 themselves over a larger extent of country. They 

 solve the problem of food supply and demand by 

 these annual pilgrimages to the boreal lands of 

 plenty. 



To go a little more to the root of the matter, we 

 may easily imagine how the migrating spirit got its 

 first impulse and gradually became evolved into a 

 habit of something like scientific precision. If the 

 first birds lived in tropical climates, as was probably 

 the case, some of them, as the food supply became 

 exhausted, would be crowded northward, or would 

 go of their own accord, and wherever they went 

 they would find well-filled natural larders. Having 

 once discovered that spring replenished the north 

 with food, they would soon learn the desirability of 

 making periodical journeys to that part of the globe. 

 With this constant quest for food must also be 

 coupled the instinctive desire of most birds for 

 seclusion during the season of reproduction, — an 

 instinct that would naturally drive them northward 

 into the less thickly tenanted districts. But it may be 

 objected that many species make long aerial voyages, 

 passing over vast tracts of country to reach their 

 chosen summer habitats in various parts of the 

 north ; and it is well known that the same individ- 

 uals will return again and again, on the recurrence 

 of spring, to the same locality. How are these facts 

 to be accounted for? 



If we accept the glacial theory — a hypothesis 

 pretty well established now among scientific men ' 



