STRUGGLE FOR EXISTENCE S3 



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 re- 

 flects 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 re- 

 marks, 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 in- 

 stance, 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 vigor- 

 ous, though fully grown plants; thus, out of twenty species grown 

 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 ex- 

 treme 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 number 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 



