Survival of the Fittest. 431 
own eggs or young, a small number may be produced, and 
the average stock be kept up; but if many eggs or young 
are destroyed, then many must be produced or the species 
will become extinct. Therefore, the average number of any 
animal or plant depends, though only indirectly, upon the 
number of its eggs or seeds. We should never forget, in 
taking a survey of nature, that every single organic being 
around us may be said to be striving to the utmost to aug- 
ment its members; that each lives by a struggle at some 
period of its existence, and that heavy destruction falls either 
on the young or old during each generation or at recurrent 
intervals. Let any check be lightened, or the destruction be 
mitigated ever so little, and the number of the species will 
almost instantaneously increase to any extent. 
But of the nature of the checks to increase we know little, 
although this subject has been very ably treated by writers 
of eminence. Eggs or very young animals seem genetally 
to suffer the most, but this is not invariably the case. 
While there is a vast destruction of the seeds of plants, but 
it is the seedlings which are believed to suffer the greatest, 
from germinating in ground already thickly stocked with 
other plants, and from being destroyed in large numbers by 
various enemies. The amount of food for each species of 
course determines the extreme limit to which each can 
increase, but very often it is not the obtaining of food, but the 
serving as prey to other animals which fixes the average 
number of a species. Thus there seems to be little doubt 
that the stock of partridges, grouse and hares on any large 
estate depends mainly on the destruction of vermin. Were 
not a single head of game shot during the next twenty 
years in England, says Darwin in substance, and no ver- 
min were at the same time destroyed, there would in 
all probability be less game than at present exists, although 
hundreds of thousands of game animals are now annually 
killed for the market. In some cases, on the other hand, 
as in the case of the elephant, none are destroyed by 
