32 o:r THE TENDENCY OF VARIETIES TO DEPAIZT 



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 



