CiiAr. III. NATURE OF THE CHECKS TO INCREASE. 75 



on a jiicco of ground three feet long and two wide, dug and 

 cleared, and wliere there could be no choking from other plants, 

 I marked all the seedlings of our native weeds as they came 

 up, and out of the 357 no less than 295 were destroyed, chielly 

 by slugs and insects. If turf which has long been mown, and 

 the case would be the same with turf closely browsed by quad- 

 rupeds, 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 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 par- 

 tridges, 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, tliere would, in all 

 prnba])ility, be less game tlian at present, although hundreds 

 of thousands of game animals are now annually killed. On the 

 other hand, in some eases, as with the elephant, none are de- 

 stroyed by beasts of prey; for even the tiger in India most 

 larely dares to attack a young elephant protected by its dam. 



Climate plays an important part in determining the aver- 

 ige numbers of a species, and periodical seasons of extreme 

 cold or drought seem to be the most effective of all checks. I 

 estimated (chiefly from the greatly reduced numbers of nests 

 in the sjiring) 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 ex- 

 traordinarily 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 cliiefly 

 acts in reducing food, it brings on the most severe struggle 

 between the individuals, whether of the same or of distinct 

 species, which subsist on the same kind of food. Evon when 

 climate, for instance extreme cold, acts directly, it will be the 

 least vigorous, or thost; whicli have got least i'ood through the 

 advancing winter, which will suffer most. "When we travel 

 from south to north, or from a damp region to a dry, we in- 

 variidily sec some species gradually getting rarer and rarer, 



