rives 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 still deficient in a constant and abundant
supply of wholesome food. Those whose organization does not permit them
to migrate when their food becomes periodically scarce, can never attain
a large population. This is probably the reason why woodpeckers are
scarce with us, while in the tropics they are among the most abundant of
solitary birds. Thus the house sparrow is more abundant than the
redbreast, because its food is more constant and plentiful,--seeds of
grasses being preserved during the winter, and our farm-yards and
stubble-fields furnishing an almost inexhaustible supply. Why, as a
general rule, are aquatic, and especially sea birds, very numerous in
individuals? Not because they are more prolific than others, generally
the contrary; but because their food never fails, the sea-shores and
river-banks daily swarming with a fresh supply of small mollusca and
crustacea. Exactly the same laws will apply to mammals. Wild cats are
prolific and have few enemies; why then are they never as abundant as
rabbits? The only intelli