HABITATS 39 



(especially where parasites are concerned) the associ- 

 ated animals : how animals are enabled to find the 

 type of habitat in which they are best suited to Uve. 

 This problem leads on in turn to the question of 

 migration and dispersal. It will be seen that these 

 subdivisions of the subject of physical habitats form 

 a natural sequence which would be followed in build- 

 ing up a complete explanation of the distribution of 

 animals. Having accurately defined the habitats, 

 we must describe the animal communities which live 

 in them, and then seek an explanation of the action 

 of the physical habitat factors in hmiting distribution 

 and numbers, while not forgetting that every animal 

 species must possess some means of finding a suitable 

 habitat for itself, in doing which dispersal is usually 

 necessary. 



The study of physical habitats has partly become 

 spht into separate sciences, as in the case of meteor- 

 ology and to some extent of oceanography and soil 

 surveys. Such sciences do not always give the data 

 required in ecological work. Meteorology has aimed 

 mainly at understanding the physical complex of the 

 atmosphere. While it had obtamed generalizations 

 of value in ecology, such as the action of depressions 

 as units of cHmate, and broad averages for the 

 temperature and rainfall of various regions, it usually 

 happens that the animal ecologist finds the methods 

 of meteorology more valuable than the results. In 

 addition, certain important techniques have been 

 evolved by animal and plant ecologists to suit their 

 work. Among these may be mentioned the Living- 

 ston atmometer for measurement of rates of evapora- 

 tion, methods of obtaining the total radiant energy 

 striking a black surface, for getting humidities in 

 very small spaces such as holes in the ground and 

 in walls, for measuring the temperature under bark 

 and at various depths in tree trunks, etc. These and 

 other methods are fully described by Shelford (1929), 

 Chapman (1931), Buxton (1931, 1932), and others. 



