152 
Allen on the Instinct of Migration. 
If what is here given as conjectural can be shown to be in part 
actual, and, as to the rest, eminently probable, the hypothesis must 
be largely strengthened. In the first place, it may be safely as- 
sumed that migratory birds can breed securely only within isotherms 
corresponding nearly with those which now limit their breeding 
ranges. From the nature of the case, demonstration of this is at 
present impossible ; but every inference that can be drawn from the 
phenomena of their distribution leads to this conclusion, as, for ex- 
ample, the occurrence of Arctic-breeding species as summer resi- 
dents of isolated alpine regions far south of their usual breeding 
limit. Furthermore, the “struggle for existence” maybe supposed 
to enforce occupation of all the available breeding area. It is also 
noteworthy that the food of migratory species is almost wholly, or 
at least in large part, insectivorous, or consists of insects and such 
soft fruits as last for an equally short period, while that of sedentary 
species living in high latitudes is of such a varied character that the 
supply is almost equally sure at all seasons. 
As to the second proposition, nothing can well be more certain 
than that migratory species breeding in high latitudes would, almost 
without exception, perish from the failure of food, to say nothing of 
the direct effect of what would in most cases prove to be fatal cli- 
matic changes, should they attempt to winter at their breeding areas. 
Again, what is predicted as 'probable in respect to the present coin- 
cidence of the “breeding and subsistence areas,” and the links that 
may connect complete coincidence with complete separation of these 
areas, is an actuality susceptible of almost numberless illustrations. 
Indeed, such conditions often exist in one and the same species, 
many examples of which may doubtless be cited from among the 
birds of almost any country, embracing a wide range of latitude. 
The birds of the United States afford probably at least a dozen 
species, the representatives of which are migratory over the north- 
ern portion and sedentary along the southern portion of their re- 
spective habitats. Prominent among such are the Meadow Lark, 
the Purple Grackle, the Red-winged Blackbird, the Towhee Bunting, 
and the Bluebird. The gist of the whole matter, however, lies in 
the following. “Now,” says Mr. Wallace, “if we suppose that the 
two areas were (for some remote ancestor of the existing species) 
coincident, but by geological and climatic changes gradually di- 
verged from each other, we can easily understand how the habit of 
incipient and partial migration at the proper seasons would at last 
