Chap. III.] NATURE OF THE CHECKS TO INCREASE. 55 



check, mitigate the destruction 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 species to 

 mcrease 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 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 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 o\ 

 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. 



The amount of food for each species of course gives the extreme 

 limit to which each can increase ; but very frequently it is not the 

 obtaining food, but the serving as prey to other animals, which 

 determines the average numbers of a species. Thus, there seems to 

 be little doubt that the stock of partridges, grouse, and hares on 

 any large estate depends chiefly on the destruction of vermin. If 

 not one head of game were shot during the next twenty years in 

 England, and, at the same time, if no vermin were destroyed, there 

 would, in all probability, be less game than at present, although 

 hundreds of thousands of game animals are now annually shot. On 

 the other hand, in some cases, as with the elephant, none are 



