TENDENCY OF SPECIES TO FORM VARIETIES. 15 



must perish annnaUy, — a striking result, but one which seems at least 

 highly probable, and is perhaps under rather than over the truth. It 

 would therefore appear that, as far as the continuance of the species 

 and the keeping up the average number of individuals are concerned, 

 large broods are superfluous. On the average all above one become 

 food for hawks and kites, wild cats and 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 ex- 

 planation 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 con- 

 dition 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 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 abun- 

 dant 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 

 abundant than the red-breast, 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 sup- 



