J. 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 
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