CHARACTERISTICS OF ASSOCIATIONS 7 



we recognise three factors which have a most profound 

 eifect upon the plant and animal organism. These are : 

 (i) the medium which surrounds the organism, either air 

 or water. This determines the method of breathing, and 

 with few exceptions prohibits the passage of the inhabi- 

 tants from one medium to another ; (2) the presence or 

 absence of light acting either directly {e.g. effects on 

 pigmentation, sight, etc.), or indirectly through plant 

 life ; (3) the presence or absence of a substratum (Lull, 

 1917). 



The above factors enable us at once to distinguish at 

 least three main types of habitat : terrestrial, aquatic, and 

 aerial, as we have already noticed. Even these divisions 

 are not precise : no animals are purely aerial and many, 

 as we know, are amphibious. Let us make a cursory 

 examination of each of these three habitats in turn. 



The Terrestrial Habitat. — This may be said to extend 

 from high-water mark to the summits of the highest moun- 

 tains. Many subdivisions are possible, but we will take 

 only the most obvious. Though the forms which live on 

 the surface are most familiar to us, yet a great animal populace 

 is to be found inhabiting the upper layers of the soil in 

 more or less complete darkness. We thus have two great 

 sub-realms : the phanerozoic and cryptozoic (see Willey, 

 191 1). A good illustration of a cryptozoic fauna may be 

 found in the inhabitants of subterranean caves, which are 

 distinguished by such features as loss of pigmentation, 

 reduction of eyes, modification of the digestive organs, 

 attenuation of body form, and reduction of the appendages. 

 Owing to the absence of light, cave faunas present interesting 

 analogies with those of the marine abysses, e.g. modification 

 of the visual organs, simplified colours, etc. Taking our 

 classification somewhat further, other well-marked habitats 

 which we are able to distinguish are those of mountain, 

 steppe, forest, prairie, etc. 



The Aerial Habitat. — A suitable ecological subdivision 

 of this habitat is not very obvious. It is the atmosphere 

 above land surfaces which is most utilised. Relatively 



