S6 NATURE OP THE CHECKS TO INCREASE. [Chap. IIL 



ber 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 re- 

 sist 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 num- 

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

 ervation. 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 super- 

 abundance of food at this one season, increase in num- 

 ber proportionally to the supply of seed, as their num- 

 bers are cheeked during the 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 



