INDEFINITELY FROM THE ORIGINAL TYPE. 29 



greater or less fecundity of an animal is often con- 

 sidered to be one of the chief causes of its abun- 

 dance 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 observa- 

 tion 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 ! * 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 in- 



* This is under estimated. The number would really 

 amount to more than two thousand millions! 



