26 NATURAL SELECTION 1! 
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 swarm- 
ing 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 un- 
changed, 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 the weakest—the very young, the aged, and 
the diseased—while those that prolong their existence can 
only be the most perfect in health and vigour—those who are 
best able to obtain food regularly, and avoid their numerous 
enemies. It is, as we commenced by remarking, “a struggle 
for existence,” in which the weakest and least perfectly 
organised must always succumb. 
The Abundance or Rarity of a Species dependent upon its more or 
less perfect Adaptation to the Conditions of Existence 
It seems evident that what takes place among the indi- 
viduals of a species must also occur among the several allied 
species of a group,—viz., that those which are best adapted 
to obtain a regular supply of food, and to defend themselves 
against the attacks of their enemies and the vicissitudes of the 
seasons, must necessarily obtain and preserve a superiority in 
population ; while those species which, from some defect of 
power or organisation, are the least capable of counteracting 
the vicissitudes of food-supply, etc., must diminish in numbers, 
