14 THE ECOLOGY OF ANIMALS 



up the British Isles. On many of these islands, 

 isolated at various intervals since the last Ice Age, 

 specially limited faunas are found ; some of them 

 also containing peculiar island races of animals 

 (e.g. bank voles on Skomer I., mice and wren on 

 St. Kilda, etc.). A third point, already mentioned, 

 is the importance of having reference collections to 

 back the published statements of surveys. This 

 importance cannot be too widely realized. Finally 

 out of this growing mass of survey data, certain 

 general scientific conclusions are beginning to appear. 

 To these we may now turn. 



No one working in the country can fail to be 

 impressed by the great importance of vegetation in 

 controlling the conditions under which animals live. 

 We have only to compare the conditions of tempera- 

 ture and humidity and light on a heather moor with 

 those in a pine wood, to realize this. At the same 

 time, as all animals depend ultimately on plants for 

 food, the presence of a particular species of plant tends 

 to fix certain species of animals to it, and indirectly 

 to control the presence of predatory forms and para- 

 sites. In a general way, therefore, the distribution 

 of animals is highly dependent upon the distribu- 

 tion of plants. In ear her surveys it was generally 

 assumed that each vegetation type would have 

 a community of animals more or less exclusively 

 attached to it. This general attitude was reinforced 

 by the publication in 1913 of Shelford's pioneer study 

 of the animal communities in the region of Lake 

 Michigan, in which he stressed the importance of 

 physiological reactions of animals. He defined animal 

 communities in different habitats by certain indicator 

 species which were confined to a narrow range of 

 habitat. It has now been realized by most ecologists 

 that the inter-relations of animals are at least as 

 important in determining whether they can Hve in 

 a certain habitat. A rook feeds mainly on pasture 

 and arable land, and nests and roosts in woods. 



