SO 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 

 rather 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 



