70 
CHECKS TO INCKEASE. 
Chap. HI. 
When a species, owing to highly favonrable circum¬ 
stances, increases inordinately in numbers in a small 
tract, epidemics—at least, this seems generally to occur 
with our game animals—often ensue: and here we haver 
a limiting check independent of the struggle for life. 
But even some of these so-called epidemics appear to 
be due to parasitic worms, which have from some cause, 
possibly in part through facility of diflFusion amongst 
the crowded animals, been disproportionably favoured: 
and here comes in a sort of struggle between the para¬ 
site and its prey. 
On the other hand, in many cases, a large stock of 
individuals of the same species, relatively to the num¬ 
bers of its enemies, is absolutely necessary for its pre¬ 
servation. Thus we can easily raise plenty of corn and 
rape-seed, &c., in our fields, because the seeds are in 
great excess compared with the number of bmds which 
feed on them; nor can the birds, though having a super¬ 
abundance of food at this one season, increase in num¬ 
ber proportionally to the supply of seed, as their num¬ 
bers are checked during winter: but any one who has 
tried, knows how troublesome it is to get seed from a 
few wheat or other such plants in a garden: I have in 
this case lost every single seed. This view of the neces¬ 
sity of a large stock of the same species for its preser¬ 
vation, explains, I believe, some singular facts in nature, 
such as that of very rare plants being sometimes ex¬ 
tremely abundant in the few spots where they do occur ; 
and that of some social plants being social, that is, 
abounding in individuals, even on the extreme confines 
of their range. For in such cases, we may believe, that 
a plant could exist only where the conditions of its life 
were so favourable that many could exist together, and 
thus save the species from utter destruction. I should 
add that the good effects of frequent intercrossing, and 
