Survival of the Fittest. 433 
however, we reach the Arctic regions, or explore snow- 
capped summits, or absolute deserts, we perceive the struggle 
for life to be almost exclusively with the elements. 
That climate operates mainly, but indirectly, in favoring 
other species, may be clearly seen in the prodigious numbers 
of garden plants that can thoroughly well endure our 
climate, but which can never become naturalized, inasmuch 
as they cannot compete with native vegetation nor resist 
destruction by native animals. 
When a species, owing to highly favorable conditions, 
increases inordinately in numbers in a small tract of country, 
epidemics, especially in game animals, often occur, and here 
we have a limiting check independent of the Struggle for 
Existence. But some of these so-called epidemics appear to 
be due to parasitic worms, which have from some cause, 
possibly in part through ease of diffusion among the crowded 
animals, been disproportionately favored, and here comes in 
a sort of struggle between the parasite and its more illustri- 
ous prey. 
But, on the other hand, as is frequently the case, a large 
stock of individuals of the same species, relatively to the 
number of its enemies, is absolutely essential to its preserva- 
tion. We thus see how it is possible to raise with ease a 
plentiful supply of corn in our fields, because the seeds are 
greatly in excess of the number of birds which feed thereon. 
Nor can the birds, though blessed with a superabundance of 
food at this one season, increase in number in proportion to 
the supply of seed, as their numbers are checked during the 
winter. Any one, however, who has made the experiment, 
knows how troublesome it is to get seed from a few wheat or 
other such plants sown broad-cast in a garden. Some 
singular facts in nature, such as that of very rare plants 
being sometimes extremely abundant in the few spots where 
they do occur, and that of some social plants being social, 
or abounding in individuals, even on the extreme confines of 
their range, are readily explainable by this view of the 
