/. A. ALLEN. 33 



tary impulse, as inherent and mandatory as the "instinct" 

 of reproduction. But why do birds migrate ? In consider- 

 ing this question it must be borne in mind that there is 

 everywhere a constant struggle for existence — that through- 

 out nature the birth-rate is far above the possibilities of 

 permanent increase. Hence, in the bird world, as elsewhere, 

 every station affording favorable conditions for existence 

 must be occupied ; there can be no unutilized corners. 

 Many birds are organized to subsist only upon either insects 

 or soft fruits, or upon both combined; these abound in 

 summer in regions far to the northward of where they are 

 found in winter. Thus many of our Swifts, Swallows, 

 Warblers and Flycatchers can range in summer to the very 

 borders of the Arctic Circle, where for a few weeks they find 

 an abundance of food and a congenial temperature. Here 

 they nest and rear their young, but are forced to retire at 

 the approach of autumn, retreating gradually before the 

 southward advance of the cold wave, passing through the 

 middle latitudes in September, and reaching the tropics in 

 October or November, the time varying more or less accord- 

 ing to the species. Here they remain till the increased 

 warmth of March or April awakens the procreative impulse 

 and admonishes them of the return of genial conditions 

 further northward. Then they begin to retrace the journey 

 toward their summer haunts, keeping pace so exactly with 

 the advance of the season as not to lose even a day of the 

 brief interval available for their sojourn in their semi-arctic 

 home. They are thus wanderers for three-fourths of the 

 year. Evidently our northern-breeding insectivorous and 

 berry-eating birds could not survive a winter at their breed- 

 ing grounds. They might perhaps live the whole year in 

 the tropics, and possibly in the lower temperate latitudes — 

 at the risk, however, of overcrowding the regular occupants, 

 and of also leaving a habitable area unoccupied. As a 

 matter of fact nature not only " abhors a vacuum," using the 

 phrase in a strict sense, but allows no waste places ; living 

 3 



