J. 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 tothe 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- 
