106 THE ORIGIN OP SPECIES 



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. 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 seedlings suffer most 

 from germinating in ground already thickly stocked with 

 other plants. Seedlings, also, are destroyed in vast num- 

 bers 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 insects. /' 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 species growing on a 

 little plot of mown turf (three feet by four) nine species 

 perished, from the other species being allowed to grow 

 up freely. ■ 



Tbe amount of food for each species of course gives 

 the extreme limit to which each can increase; but very 



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