MIGRATION OF BIRDS. 145 
plovers—and it seems quite a natural thing to see them 
crouching on the bleak rocks when the north-east winds or 
winter are blowing ; but it seems very unnatural to hear 
of grey plovers wintering in the tropics of Central Africa, of 
turnstones tinkling out to sea when disturbed from the 
heathen rocks of the Cape Verde Islands, Again, if soft- 
billed birds of feeble flight even journey as far as Cape 
Colony, and their species form no continuous line north- 
wards, it will be sufficient to note that when winter drove 
them south to North Africa, and further east towards 
Persia, it drove them on to that arid zone that practically 
reaches round the world. There is a vast desert tract 
that stretches from Morocco across Arabia into the Punjanb 
of India. Soft-billed birds had either to cross this arid 
region or starve. Once across it, such prolific birds as the 
willow-warblers would require a wide area, and such highly 
developed flying birds as swallows would think little of 
distances ; and lastly, not a hundred birds out of a million 
hatched in the southern hemisphere visit us—in fact, I only 
know of some shearwaters of the North Atlantic. The reason 
is, they have no need to; when winter forces them north- 
wards, it forces them into increased land areas and across no 
arid zones, 
So far it seems possible to understand what is called the 
mystery of migration, but when birds found regions of 
plenty in these tropical and sub-tropical parts, why did not 
they remain there 2 They do not remain in these quarters 
till food is exhausted, but with the signs of spring they 
move off, just as they leave their northern haunts, some of 
them, long before the food-supplies are exhausted. By way 
of explanation, first it may be noted that the lapse of ages 
had formed a definite habit that had its beginnings in very 
small things. It is easy imagining the development of 
habit. To begin with, when they were forced to migrate 
yearly, they lingered in their haunts till they could linger 
no longer—the yearly supply would not always exhaust itself 
on the very same day, and that would produce an unsettling 
effect ; they would need to strike, as it were, an average 
time for leaving, and in some way it would be noted that 
that time of leaving corresponded with decided changes of 
VOL. II. me CL 
