280 



THEORIES AS TO THE NATURE OF SPECIES, [Ch. XXXV. 



Wallace 



perish annuallj. ' Very few birds/ says Mr. 

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

 six, eight, or ten ; and if we suppose that each pair produce 

 young only four times in their life, each would at this rate 

 increase in fifteen years 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 even in 150 years. 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, 

 tAvice that number must perish annually. 



' Large broods are superfluous : on the average all above 

 one become food for hawks and kites, wild cats and weazels 



comes 



emarkable instances of an immense 



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 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 re- 

 quisite 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 sufiicient to 



t 



I 



When 



* Joiirn. of Linnoean Soc, vol. in. p. bb, 18o8i 

 t Ibid. p. 55, 



