24 NATURAL SELECTION i 
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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!! 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 millicus ! 
