110 THE ORIGIN OF SPECIES 



good eflfecta of intercrossing, cand 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 Relatioiis 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 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 inclosed twenty-five years previously 

 and planted with Scotch fir. The change in the native 

 vegetation of the planted part of the heath was most 

 remarkable, more than is generally seen in passing from 

 one quite different soil to another: not only the propor- 

 tional numbers of the heath-plants were wholly changed, 

 but twelve species of plants (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 common 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 effect of tha introduction of a single 

 tree, nothing whatever else having been done, with the 

 exception of the land having been inclosed, so that cattle 

 could not enter. But how important an element inclosure 



