chap. ill. Struggle for Existence. 55 



When a species, owing to highly favourable circumstances, 

 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 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 

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

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

 acres of exactly the same nature had been enclosed twenty-five 

 years previously and planted with Scotch fir. The change in the 



