Chap. IH.] NATUEE OF THE CHECKS TO INCEEASE. 83 



Nature of the Clucks 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 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 surfer 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 germi- 

 nating 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 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 



