6302 



Tendency of Species 



many young are much less plentiful ? The explanation is not difficult. 

 The food most congenial to this species, and on which it thrives best, 

 is abundantly distributed over a very extensive region, offering such 

 differences of suil and climate, that in one part or another of the area 

 the supply never fails. The bird is capable of a very rapid and long- 

 continued flight, so that it can pass without fatigue over the whole of 

 the district it inhabits, and as soon as the supply of food begins to 

 fail in one place is able to discover a fresh feeding-ground. This 

 example strikingly shows us that the procuring a constant supply of 

 wholesome food is almost the sole condition requisite for ensuring 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 suf- 

 ficient to check it. In no other birds are these peculiar circumstances 

 so strikingly combined. Either their food is more Hable to failure, or 

 they have not sufficient power of wing to search for it over an exten- 

 sive 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 

 w'ilh us, wdiile 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. Wildcats 

 are prolific and have few enemies ; why then are they never as abun- 

 dant 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 



