STRUGGLE FOR EXISTENCE 107 



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 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 proba- 

 bility, 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 pro- 

 tected by its dam. 



_Climate plays an important part in determining the 

 average 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 re- 

 duced 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 cli- 

 mate seems at first sight to be quite independent of the 

 struggle for existence; but in so far as climate chiefly 

 acts in reducing food, it brings on the most severe strug- 

 gle between the individuals, whether of the same or of 

 distmct species, which subsist on the same kind of food. 

 Even when climate, for instance extreme cold, acts di- 

 rectly, it will be the least vigorous individuals, or those 

 which have got least food through the advancing winter, 

 which will suffer most. When we travel from south to 



