CHAP. III.] STRUGGLE FOB EXISTENCE. 51 



in our gardens can perfectly well endure our climate, but which 

 never become naturalised, for they cannot compete with our native 

 plants nor resist destruction by our native animals. 



When a species, owing to highly favourable 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 

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

 possibly in part through facility of diffusion amongst the crowded 

 animals, been disproportionally 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 numbers of its enemies, is 

 absolutely necessary for its preservation. Thus we can easily 

 raise plenty of corn and rape-seed, &c., 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 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 favourable 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 

 to struggle together in the same country. I will give only a single 

 instance, which, though a simple one, interested me. In Stafford- 

 shire, on the estate of a relation, where I had ample means of in- 

 vestigation, there was a large and extremely barren heath, which 

 had never been touched by the hand of man ; but several acres of 

 exactly the same nature had been enclosed twenty-five years pre- 



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