1472 THE STORY OF THE UNIVERSE 



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 grow, 

 the more vigorous plants gradually kill the less vig- 

 orous, 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 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 deter- 

 mines the average numbers of 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 

 destroyed, 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 is 

 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 sea- 

 sons of extreme cold or drought seem to be the most 

 effective of all checks. I estimated (chiefly from the 



