STRUGGLE FOR EXISTENCE 55 



summits, or absolute deserts, the struggle for life is almost ex- 

 clusively 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 destruction by our native animals. 



When a species, owing to highly favorable circumstances, in- 

 creases 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 ap- 

 pear to be due to parasitic worms, which have from some cause, 

 possibly in part through facility of diffusion among the crowded 

 animals, 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 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 win- 

 ter; 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 preservation, 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 could 

 exist only where the conditions of its life were so favorable that 

 many could exist together, and thus save the species from utter 

 destruction. I should add that the good effects of intercrossing, 

 and the ill effects of close interbreeding, no doubt come into play 

 in many of these cases; but I will not here enlarge on this subject. 



COMPLEX RELATIONS OF ALL ANIMALS AND PLANTS TO 

 EACH OTHER IN THE STRUGGLE FOR EXISTENCE 



Many cases are on record showing how complex and unexpected 

 are the checks and relations between organic beings, which have 



