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 
I 
