NATUBFj of the CHEGK8 TO INCREASE. C3 



NATURE OB 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 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. Plere I will make only a 

 few remarks, just to recall to the reader's mind some'of the 

 chief point§. 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 seed- 

 lings 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 de- 

 stroyed, 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 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 extreme limit to which each can increase; but very fre- 

 quently 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 years in 



