ORIGIN OF MIGRATION, 59 
tance in search of a place where they may conceal their 
nests. 
Many species of tropical sea birds resort each year to 
some rocky islet, situated perhaps in the heart of their 
range, where they may nest in safety. This is not migra- 
tion as we understand the word; but, nevertheless, the 
object is the same as that which prompts a Plover to 
travel to the arctic regions ; moreover, the movement is 
just as regular. These sea birds pass their lives in the 
tropics, their presence or absence in any part of their 
range being largely dependent upon the supply of food. 
But, as in the case of the Warbler which migrates from 
South America to Labrador, they are annually affected 
by an impulse which urges them to hasten to a certain 
place. This impulse is periodic, and in a sense is com- 
mon to all birds. There is a regular nesting season in 
the tropics, just as there is a regular nesting season in the 
arctic regions. 
There is good reason, therefore, for the belief that 
the necessity of securing a home in which their young 
could be reared was, as it still is, the cause of migration. 
It must be remembered, however, that birds have been 
migrating for ages, and that the present conditions are 
the result of numerous and important climatic changes. 
Chief among these is doubtless the Glacial period. In- 
deed, Dr. Allen has stated, and the theory has been gen- 
erally accepted, that the migration of birds was the out- 
come of the Glacial period. Prior to those climatic 
changes which, beginning in the latter part of the Ter- 
tiary period, culminated in the ice age, a warm temperate 
or subtropical climate prevailed in the Arctic regions, 
and it was not necessary for birds to migrate. Driven 
southward by the formation of ice, many species doubtless 
became extinct. As the ice receded birds followed it 
northward, only to retreat southward each year at the 
