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

32 ON THE TENDENCY OF VARIETIES TO DEPART
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 reasons why woodpeckers are scarce with us, while in the tropics they
are among the most abundant of solitary birds.
Thus the house sparrrow 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 seashores 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 intelligible answer is, that their supply of food is
more precarious. It appears evident, therefore, that
so long as a country remains physically unchanged,
the numbers of its animal population cannot materially increase. If one species does so, some others
requiring the same kind of food must diminish in
proportion. The numbers that die annually must be
immense; and as the individual existence of each
animal depends upon itself, those that die must be