70 CHECKS TO INCREASE. Chap. III. 



Wlien 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 disproportionably favoured : 

 and here comes in a sort of struggle between the para- 

 site 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, &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 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 tins 

 case lost every single seed. This view of the necessity 

 of a large stock of the same sj)ecies 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 occur; and 

 that of some social plants being social, that is, abound- 

 ing 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 conditions of its life were so 

 favourable that many could exist together, and thus save 

 each other from utter destruction. I should add that the 

 good effects of frequent intercrossing, and the ill effects 



