TENDENCY OF SPECIES TO FOEM VABIETIES. 55 



some powerful check to their natural increase. Very lew birds 

 produce less than two young ones each year, while many have 

 six, eight, or ten ; four will certainly be below the average ; and if 

 we suppose that each pair produce young only four times in their 

 life, that will also be below the average, supposing them not to 

 die either by violence or want of food. Yet at this rate how tre- 

 mendous would be the increase in a few years from a single pair I 

 A simple calculation will show that in fifteen years each pair of 

 birds would have increased to nearly ten millions ! whereas we 

 have no reason to believe that the number of the birds of any 

 country increases at all in fifteen or in one hundred and fifty years. 

 With such powers of increase 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, therefore, 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 existing 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 appear that, as far as the con- 

 tinuance of the species and the keeping up the average number 

 of individuals are concerned, large broods are superfluous. On 

 the average all above one become food for hawks and kites, wild 

 cats and 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 re- 

 markable 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 plen- 

 tiful ? The explanation is not difficult. The food most congenial 

 to this species, and on which it thrives best, is abundantly distri- 

 buted 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-contiuued 

 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 



