ii ON THE TENDENCY OF VARIETIES, ETC. 25 



food for hawks and kites, wild cats or weasels, or perish of 

 cold and hunger as winter comes on. This is strikingly 

 proved by the case of particular species; for we find that 

 their abundance in individuals bears no relation whatever to 

 their fertility in producing offspring. 



Perhaps the most remarkable instance of an immense bird 

 population is that of the passenger pigeon of the United 

 States, which lays only one, or at most two eggs, and is said 

 to rear generally but one young one. Why is this bird so 

 extraordinarily abundant, while others producing two .or three 

 times as many young are much less plentiful ? The explana- 

 tion 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 soil 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 un- 

 restrained 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 organisation does not permit 

 them to migrate when their food becomes periodically scarce, 

 can never attain a large population. This is probably the 



