STRUGGLE FOR EXISTENCE 65 



stances, 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 

 amongst the crowded animals, been disproportionably 

 favoured : 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 num- 

 bers of its enemies, is absolutely necessary for its pre- 

 servation. 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 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 preservation, explains, I believe, some singular facts 

 in nature, such as that of very rare plants being some- 

 times extremely abundant in the few spots where they 

 do occur ; and that of some social plants being social, 

 that is, abounding in individuals, even on the extreme 

 confines of their range. For in such cases, we may 

 believe, that a plant could exist only where the con- 

 ditions of its life were so favourable that many could 

 exist together, and thus save the species from utter 

 destruction. I should add that the good effects of 

 frequent intercrossing, and the ill effects of close inter- 

 breeding, probably come into play in some of these cases; 

 but on this intricate subject I will not here enlarge. 



Many cases are on record showing how complex and 

 unexpected are the checks and relations between organic 

 beings which have to struggle together in the same 



F 



