30 ON THE TENDENCY OF VARIETIES TO DEPART 
crease the population must have reached its limits, 
and have become stationary, in a very few years 
after the origin of each species. It is evident, there- 
fore, that each year an immense number of birds 
must perish—as many in fact as are born; and as 
on the lowest calculation the progeny are each year 
twice as numerous as their parents, it follows that, 
whatever be the average number of individuals exist- 
ing in any given country, twice that number must 
perish annually,—a striking result, but one which. 
seems at least highly probable, and is perhaps under 
tather than over the truth. It would therefore ap- 
pear that, as far as the continuance of the species 
and the keeping up the average number of indi- 
viduals are concerned, large broods are superfluous. 
On the average all above one become 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 off- 
spring. 
Perhaps the most remarkable instance of an im- 
mense 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 gener- 
ally but one young one. Why is this bird so 
extraordinarily abundant, while others producing two 
or three times as many young are much less plen- 
tiful? The explanation is not difficult. The food 
