lL 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 
