56 MESSRS. C. DARWIK AND A. WALLACE ON THE 



of wholesome food is almost the sole condition requisite for ensu- 

 ring the rapid increase of a given species, since neither the limited 

 fecundity, nor the unrestrained attacks of birds of prey and of 

 man are here sufficient to check it. In no other birds are these 

 peculiar circumstances so strikingly combined. Either their food 

 is more liable to failure, or they have not sufficient power of wing 

 to search for it over an extensive area, or during some season of the 

 year it becomes very 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, can 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 abun- 

 dant 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 inexhaust- 

 ible supply. Why, as a general rule, are aquatic, and especially 

 sea birds, very numerous in individuals ? JSTot 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 moUusca 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 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 



