ECOLOGICAL SURVEYS 15 



There is no suggestion that the cHmatic conditions 

 in either of these types of habitat greatly influence 

 its choice. In the one instance the rook goes for its 

 food, in the other it goes to a high place where ground 

 enemies cannot reach its young, or itself while asleep. 

 A parasitic Tachinid fly or Chalcid wasp cannot live 

 and breed unless appropriate hosts are there. The 

 herring follows the plankton, and so do whales. 



In analysing the lists of animals of different major 

 habitats that result from ecological surveys, we find 

 that a large number of species are not confined to 

 one fife zone or vegetation type or to any narrow 

 set of physical or even biological conditions. At the 

 same time each animal community presents charac- 

 teristic features which enable us to speak of the 

 community of an oak wood or of a sand dune. The 

 characteristic facies that we meet with is given to 

 it by several different features. First there are the 

 animals that are exrlusively confined to the community, 

 and these often have special adaptations for fife in 

 such places. An example is the nutcracker {Nuci- 

 fraga), a bird of northern Europe and Siberia, whose 

 beak is adapted for the sole purpose of breaking the 

 seeds of cedar trees. The Siberian race has a thimier 

 beak than the European race, and this is correlated 

 with the geographical races of cedar, the eastern 

 having a thin shell and the western a thick shell 

 (Formosof, 1933). Secondly, some animals, although 

 occurring elsewhere, are more abundant in the com- 

 munity than outside it. Wood-mice {Apodemus) are 

 extremely common in woods, though also found 

 outside them, in gardens and hedgerows and occa- 

 sionally in fields. Finally there is another thing that 

 gives a characteristic appearance to an animal com- 

 munity. The animals stand in a special relation to 

 one another ; that is, the community has a certain 

 organization or structure. This would be caUed its 

 economic structure, if we were studying a human 

 community. The second class of animals, those 



