Contributions to the theory of natural selection. A series of essays ... By Alfred Russel Wallace ...

INDEFII~TELY FROM THE ORIGINAL TYPE. 31
most congenial to this species, and on which it
thrives best, is abundantly distributed over a very
extensive region, offering such differences of soil
and climate, that in one part or another of the
area the supply never fails. The bird is capable of
a very rapid and long-continued flight, so that it
can pass without fatigue over the whole of the district it inhabits, and as soon as the supply of food
begins to fail in one place is able to discover
a fresh feeding-ground. This example strikingly
shows us that the procuring a constant supply of
wholesome food is almost the sole condition requisite for ensuring the rapid increase of a given
species, since neither the limited fecundity, nor the
unrestrained attacks of birds of prey and of man
are here sufficient to check it. In no other birds
are these peculiar circumstances so strikingly combined. Either their food is more liable to failure,
or they have not sufficient power of wing to search
for it over an extensive area, or during some
season of the year it becomes very scarce, and less
wholesome substitutes have to be found; and thus,
though more fertile in offspring, they can never increase beyond the supply of food in the least
favourable seasons.
Many birds can only exist by migrating, when
their food becomes scarce, to regions possessing a
milder, or at least a different climate, though, as
these migrating birds are seldom excessively abundant, it is evident that the countries they visit are