y. A. ALLEN. 35 



not so obvious, since in most instances it cannot be due to 

 a failure of the food supply, nor to any absolute incompat- 

 ibility of climate. We are forced then to conclude that it 

 is due partly to a habit of such long standing that it has 

 become irresistible, or in a certain sense a part of the bird's 

 organism, but partly, and perhaps mainly, to the necessity of 

 returning to a region to which it has become so thoroughly 

 adapted as to be indispensable to its well-being during the 

 season of procreation. The return to the breeding station 

 in spring has often been attributed to " strong home-love." 

 That this home-love exists is shown by the return of birds 

 to the same locality — even to the same nesting-place — for 

 many successive years, of which there is so much proof that 

 it is commonly assumed to be the rule in most species. It is 

 certainly beyond question that birds do not select their 

 breeding haunts in any haphazard way, journeying north 

 along a vague course and stopping to nest wherever the 

 proper conditions of season and other surroundings happen to 

 prove favorable. Hence the impulse that governs their spring 

 movements has been often loosely termed the " home instinct." 



As already shown, the impulse to migrate, or rather the 

 habit of migration itself, must have originated ages ago, as 

 the result of a profound change in the climatic conditions 

 of the earth following the close of the Tertiary period, and 

 that through the lapse of thousands of centuries the habit of 

 migration has passed down from generation to generation 

 till it has become hereditary — as much so as any other trait — 

 as that of nest-building, for example, in respect to choice of 

 materials and the peculiar architectural effects characteristic 

 of different species ; or of laying eggs with distinctive color- 

 markings, etc. 



The subject of migration has perhaps been rendered need- 

 lessly complicated by considering the vernal and autumnal 

 movements separately, and trying to find a different and 

 special cause for each. A complete cycle of migration con- 

 sists necessarily of two movements — from the breeding sta- 



