WHY DO BIRDS MIGRATE? 
57 
have been crowded thence toward the tropics; and the struggle for 
life thereby greatly intensified. The less yielding forms may have 
become extinct; those less sensitive to climatic changes would seek to 
extend the boundaries of their range by slight removal northward 
during the milder intervals of summer, only, however, to be forced 
back again by the recurrence of winter. Such migration must have been 
at first ‘incipient and gradual/ extending and strengthening as the 
cold-wave Ice Age receded and opened up a wider area within which 
existence in summer became possible. What was at first a forced migra- 
tion would become habitual, and through the heredity of habit give 
rise to the wonderful faculty, which we call migration.” 
While it is by no means certain that “the necessity of migration” 
did not exist prior to the Glacial Period, it seems probable that, whether 
or not this period actually gave rise to bird migration, it affected the 
movements of birds much as Allen has suggested. 
It is to the influence of the Glacial Period that we must attribute 
the presence, in the warmer parts of the globe, today, of such physi- 
cally closely related, but geographically widely separated birds as 
Ibises, Spoonbills, Pelicans, Frigate Birds, Anhingas and Flamingoes. 
It is out of the question to believe that birds, so like each other and 
so unlike other birds, can have originated independently in the old 
world and in the new; whence it follows, of course, that they have 
descended from a common ancestor, or, in other words, that their 
ranges were at one time continuous.' This time we may well believe 
to have been that portion of the Tertiary Period preceding the Glacial 
Epoch, when the warm climate of the polar regions was adapted to 
their wants. With the climatic change which culminated in the Ice 
Age, their boreal representatives either became extinct or were forced 
southward, some in the Old World, some in the New, and the territory 
thus deserted has never been reacquired. The White Pelicans, and 
many other species closely related to Old World forms, and now breed- 
ing north of the most southern limit of the great Ice Field (for instance, 
the Eared Grebe, Gannet, Great Blue Heron, Gallinule, Oyster-catcher, 
Crossbill and Brown Creeper), have, however, evidently extended their 
summer ranges to the northward of the area which they occupied 
during the maximum development of the Ice Age. To speak of only 
the White Pelican, the reasons have just been stated for believing 
this species to have formerly inhabited the shores of the Arctic Ocean, 
whence it was forced southward, below possibly the fortieth degree 
of latitude, by the rigors of the climate of the Ice Age. Nevertheless 
it now breeds regularly as far north as latitude 61 °, and has therefore 
regained at least a thousand miles in latitude, of the region from 
which it has been forced; but each year the individual repeats the 
history of the species, by retreating before what may be termed a 
seasonal Ice Age, as winter seals the lakes and rivers on which it has 
passed the summer. 
In a similar manner the migrations of each of our birds may be 
