the foregoing considerations always in mind — never to forget that every single organic being may be said to be striving to the utmost to increase in numbers; that each lives by a struggle at some period of its life; that heavy destruction inevitably falls either on the young or old during each generation or at recurrent intervals.

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

4. Nature Of The Checks To Increase

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