Chap. III. STRUGGLE FOR EXISTENCE. 77 



a superabundance of food at tliis one season, increase in num- 

 ber 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 some- 

 times extremely abundant in the few spots where they do 

 occur; and that of some social plants being social, that is, 

 abounding 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 favorable 

 that many could exist together, and thus save the species from 

 utter destruction. I should add that the good effects of fre- 

 quent intercrossing, and the ill eff'ects of close interbreeding, 

 probably come into play in some of these cases ; but on tliis 

 intricate subject I will not here enlarge. 



Complex Relations of all Animals and Plants to each other 

 in the Struggle for Existence. 



Many cases are on record showing how complex and unex- 

 pected 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, has 

 interested me. In Staffordshire, 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 native vegeta- 

 tion of the ])lanted part of the heath was most remarkable, 

 more than is generally seen in passing from one quite diiTerent 

 soil to another; not only the proportional numbers of the heath- 

 plants were wholly changed, but twelve species of j)lants (not 

 counting grasses and carices) flourished in the plantations, 

 which could not be found on the heath. The effect on the 

 insects must have been still greater, for six insectivorous birds 

 were very conmion in the plantations, which were not to be 

 seen on the heath ; and the heath was frequented by two or 

 three distinct insectivorous birds. Here we see how potent 

 has been the cff"cct of the introduction of a single tree, nothing 



