1474 THE STORY OF THE UNIVERSE 



we travel northward, but in a somewhat lesser de- 

 gree, for the number of species of all kinds, and 

 therefore of competitors, decreases northward ; hence 

 in going northward, or in ascending a mountain, we 

 far oftener meet with stunted forms, due to the di- 

 rectly injurious action of climate, than we do in pro- 

 ceeding southward or in descending a mountain. 

 When we reach the Arctic regions or snow-capped 

 summits, or absolute deserts, the struggle for life is 

 almost exclusively with the elements. 



That climate acts in main part indirectly by fa- 

 voring other species, we clearly see in the prodigious 

 number of plants which in our gardens can per- 

 fectly well endure our climate, but which never be- 

 come naturalized, for they can not compete with our 

 native plants nor resist destruction by our native 

 animals. 



When a species, owing to highly favorable cir- 

 cumstances, 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 have 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 diffusion among the crowded ani- 

 mals, been disproportionally favored: and here 

 comes in a sort of struggle between the parasite and 

 its prey. 



On the other hand, in many cases, a large stock 

 of individuals of the same species, relatively to the 

 numbers of its enemies, is absolutely necessary for 



