78 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 - 
