ORIGIN OP MIGRATION. 59 



tance in search of a place where they may conceal their 

 nests. 



Many species of tropical sea birds resort each year to 

 6ome 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. That their journeys were 

 greatly increased and the habit of migration extended 

 during the ice age is apparently undeniable, but, although 

 previous to the Glacial period a warm temperate climate 

 prevailed nearly to the northern pole, there must even 

 then have been sufficient difference between winter and 

 summer climate to render a certain amount of migration 

 necessary. Furthermore, there is a well-defined migra- 



