NATURE OF THE CHECKS TO INCREASE. r,5 



competitors, decreases northward, or in ascending a mount- 

 ain, we far oftener meet with stunted forms, due to the 

 directly injurious action of climate, than we do in proceed- 

 ing southward or in descending a mountain. When we 

 reach the Arctic regions, or snow-capped summits, or abso- 

 lute deserts, the struggle for life is almost exclusively with 

 the elements. 



That climate acts in main part indirectly by favoring 

 other species we clearly see in the prodigious number of 

 plants which in our gardens can perfectly well endure our 

 climate, but which never become naturalized, for they 

 cannot compete with our native plants nor resist destruc- 

 tion by our native animals. 



When a species, owing to highly favorable circumstances, 

 increases inordinately in numbers in a small tract, epidem- 

 ics — 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 animals, 

 but 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 indi- 

 viduals of the same species, relatively to the numbers of its 

 enemies, is absolutely necessary for its preservation. Thus 

 we can easily raise plenty of corn and rape-seed, etc., in 

 our fields, because the seeds are in great excess compared 

 with the number of birds which feed on them; nor can the 

 birds, though having a superabundance of food at this one 

 season, increase in number proportionally to the supply of 

 seed, as their numbers are checked during the 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 

 necessity 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 extremely 

 abundant, in the few spots where they do exist; and that 

 of some social plants being social, that is abounding in 

 individuals, even on the extreme verge of their range. For 

 in such cases, we may believe, that a plant conld exist only 



