ECOLOGY 57 



great numbers of individuals which completely cover 

 the ground, but there are comparatively few species. 



In every case the plants of a spot depend to an enor- 

 mous extent on the soil. Many species are exceedingly 

 sensitive to very small traces of such compounds as 

 lime, silicates, salt, &c. Some can only live when 

 supplied with lime or chalk, which to others is well- 

 nigh a poison. It is well known that the Orchids and 

 other plants which grow on the chalk downs cannot 

 live on the quartz sand of an old dune. 



In a country so much cultivated as England, however, 

 it is often difficult to see the direct influence of the soil 

 on the communities of plants growing on it, for hardly 

 any of the fields which form so great a part of the land 

 have not been subjected many times to manuring and 

 planting and to the weeding out of the original in- 

 habitants, either entirely or in part. 



The seashores, with their salt-marshes and sand- 

 dunes, and the freshwater ponds, where the land plants 

 are encroaching, are, perhaps, the best illustrations of 

 natural communities which are available for ecological 

 study in these islands. 



