152 Allen on tke InstAnd 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. Fi'om 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 " may be 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 ecpially 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 dix'ect 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 I'epresentatives 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 gi'adually di- 

 verged fi'om each other, we can easily understand how the habit of 

 incipient and partial migration at the proper seasons would at last 



