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.

This subject of the checks to increase has been ably treated by several authors, and I hope in a future work to discuss it at considerable length, more especially in regard to the feral animals of South America.

Here I will make only a few remarks, just to recall to the reader's mind some of the chief points.