Page - 82 - in The Origin of Species

82 . ORIGIN OF SPECIES falls either on the young or old, during each generation or at recurrent intervals. Lighten any check, mitigate the de- struction ever so little, and the number of the species will almost instantaneously increase to any amount. NATURE OF THE CHECKS TO INCREASE The causes which check the natural tendency of each spe- cies to increase are most obscure. Look at the most vig- orous 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 incom- parably 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 con- siderable length, more especially in regard to the feral ani- mals of South America. Here I will make only a few re- marks, just to recall to the reader's mind some of the chief points. Eggs or very young animals seem generally to suffer most, but this is not invariably the case. With plants there is a vast destruction of seeds, but, from some observations which I have made, it appears that the seedings suffer most from germinating in ground already thickly stocked with other plants. Seedlings, also, are destroyed in vast numbers by various enemies ; for instance, on a piece of ground three feet long and two wide, dug and cleared, and where there could be no choking from other plants, I marked all the seedlings of our native weeds as they came up, and out of 357 no less than 295 were destroyed, chiefly by slugs and in- sects. If turf which has long been mown, and the case would be the same with turf closely browsed by quadrupeds, be let to grow, the more vigorous plants gradually kill the less vigorous, though fully grown plants ; thus out of twenty spe- cies growing on a little plot of mown turf (three feet by four) nine species perished, from the other species being al- lowed to grow up freely. The amount of food for each species of course gives the extreme limit to which each can increase; but very fre-