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 popu- 
lation. This is probably the reasons why wood- 
peckers 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 con- 
trary; 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 in- 
telligible 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 ma- 
terially 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 
