100 Darwin- Wallace Celebration. 



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 

 continuance 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, \\ild 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 

 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 explanation 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 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 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 



