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. Hither 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 Dirds 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 
