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 prodtice 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 twe thousand millions! 
