INDEFINITELY FROM THE ORIGINAL TYPE. 31 



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 dis- 

 trict 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 re- 

 quisite 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 com- 

 bined. 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 in- 

 crease 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 abun- 

 dant, it is evident that the countries they visit are 



