24 NATURAL SELECTION n 



the more luxuriant prairies and pampas of America. The 

 greater or less fecundity of an animal is often considered to 

 be one of the chief causes of its abundance or scarcity ; but a 

 consideration of the facts will show us that it really has little 

 or nothing to do with the matter. Even the least prolific of 

 animals would increase rapidly if unchecked, whereas it is 

 evident that the animal population of the globe must be 

 stationary, or perhaps, through the influence of man, decreasing. 

 Fluctuations there may be ; but permanent increase, except in 

 restricted localities, is almost impossible. For example, our 

 own observation must convince us that birds do not go on 

 increasing every year in a geometrical ratio, as they would 

 do were there not some powerful check to their natural 

 increase. Very few 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 tremendous 

 would be the increase in a few years from a single pair ! A 

 simple calculation will show that in fifteen years each pair of 

 birds would have increased to nearly ten millions ! l 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, so 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 



1 This is under estimated. The number would really amount to more 

 than two thousand millions ! 



