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- 



