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 th'e 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 



