CHAP. HI.] NATURE OP THE CHECKS TO INCREASE. 49 



average number of any animal or plant depends only indirectly on 

 the number of its eggs or seeds. 



In looking at Nature, it is most necessary to keep the foregoing 

 considerations always in mind never to forget that every single 

 organic being may be said to be striving to the utmost to increase 

 in numbers ; that each lives by a struggle at some period of its 

 life ; that heavy destruction inevitably falls either on the young 

 or old, during each generation or at recurrent intervals. Lighten 

 any 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 

 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 man- 

 kind, 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 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 uf 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 bb little doubt that the stock of partridges, grouse, and hares 

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



