Darwin's Artificial and Natural Selection 113 



induce other forms to feed on it. Crowding will also give 

 an opportunity for the spread of disease, which again may 

 check the increase. Sooner or later a sort of ever shifting 

 balance will be reached for each species, and after this, if the 

 conditions remain the same, the number of individuals will 

 keep approximately constant. 



Darwin admits that the " causes which check the natural 

 tendency of each species to increase are most obscure." " We 

 know not exactly what the checks are even in a single in- 

 stance." This admission may well put us on our guard 

 against a too ready acceptation of a theory in which the whole 

 issue turns on just this very point, namely, the nature of the 

 checks to increase. Darwin gives the following general cases 

 to show what some of the checks to increase are. He states 

 that eggs and very young animals and seeds suffer more 

 than the adults ; that " 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 num- 

 bers of a species. Thus, there seems to be little doubt that 

 the stock of partridges, grouse, and hares on any large estate 

 depends largely on the destruction of the vermin." " On the 

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

 stroyed 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 number of a species, and periodical seasons of 

 extreme cold or drought seem to be the most effective of all 

 checks." "The action of climate seems at first sight to be 

 quite independent of the struggle for existence ; but in so far 

 as climate 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." 



We need not follow Darwin through his account of how 

 complex are the relations of all animals and plants to one 



