86 NATUKE OF THE CHECKS TO INCBEASE. [Chap. HI 



of plants which 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 circum- 

 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 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 



