Lighten any check, mitigate the destruction ever so little, and the number of the species will almost instantaneously increase to any amount.

The causes which check the natural tendency of each species to increase are most obscure.

Look at the most vigorous species; by as much as it swarms in numbers, by so much will it tend to increase still further.

We know not exactly what the checks are even in a single instance.

Nor will this surprise any one who reflects how ignorant we are on this head, even in regard to mankind, although so incomparably better known than any other animal.