Darwin- Wallace Celebration. 101 



during some season of the year it becomes \ery 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, 

 oan 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 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 im- 

 mense ; 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 organized must always succumb. 

 Now it is clear that what takes place among the individuals 

 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 



