Chap. III. CHECKS TO INCREASE. 67 



destruction ever so little, and the number of the species 

 will almost instantaneously increase to any amount. 

 The face of Nature may be compared to a yielding 

 surface, with ten thousand sharp wedges packed close 

 together and driven inwards by incessant blows, some- 

 times one wedge being struck, and then another with 

 greater force. 



What checks the natural tendency of each species to 

 increase in number is most obscure. Look at the most 

 vigorous species ; by as much as it swarms in numbers, by 

 so much will its tendency to increase be still further in- 

 creased. We know not exactly what the checks are in 

 even one single instance. Nor will this surprise any one 

 who reflects how ignorant we are on this head, even in 

 regard to mankind, so incomparably better known than 

 any other animal. This subject has been ably treated by 

 several authors, and I shall, in my future work, discuss 

 some of the checks 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, I believe that it is the seedlings which 

 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 the 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, 



