60 NATURE OF THE CHECKS TO INCREASE. 



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 tour), 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 

 England, and, at the same time, if no vermin were de- 

 stroyed, 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 destroyed by beasts of prey ; 

 for even the tiger in India most rarely dares to attack a 

 young elephant protected by its dam. 



Climate plays an important part in determining the 

 average numbers of a species, and periodical seasons of ex- 

 treme cold or drought seem to be the most effective of 

 all checks. I estimated (chiefly from the greatly reduced 

 numbers of nests in the spring) that the winter of 1854-55 

 destroyed four fifths of the birds in my own grounds; and 

 this is a tremendous destruction, when we remember that 

 ten per cent is an extraordinarily severe mortality from 

 epidemics with man. The action of climate seems at first 

 sight to be quite independent of the struggle for existence ; 

 but in so far as climate chiefly acts in reducing food, 

 ^t brings on the most severe struggle between the indi- 

 viduals, whether of the same or of distinct species, which 



