GRASSHOPPERS IX RELATION TO PLANT ASSOCIATIONS. 155 



at one end of which the conditions are those of sterile soil or 

 water, environmental control being entirely physical; at the 

 other end conditions are those of the closed, completely developed, 

 climatic association, vegetational control of local environment 

 being nearly complete. 



It is further possible to arrange the lines representing the 

 different successlonal series within a region, so that relations 

 between them may also be seen graphically. Fig. I (p. 156) is an 

 arrangement of the gradients of physical and vegetational con- 

 ditions in the Douglas Lake region, based on the development of 

 vegetation, and accompanied by a representation of the selec- 

 tion of these conditions by the various grasshopper species. 



IV. GENERAL DISCUSSION. 



The Assignment of Terrestrial Animals to Habitats. 



It is thought by some zoologists that the local distribution of 

 most terrestrial animals is more or less haphazard, that there is 

 no order in the distribution of animals into different habitats, or 

 that, if there is order, the conditions of distribution are too 

 complicated to be determined by any present methods. Shel- 

 ford ('n: 591) points out several reasons for the prevalence of 

 these and similar opinions. On the contrary, environmental 

 relations of animals are now coming to be recognized as quite 

 definite (Shelf ord, 'i2b: 333). 



The habitat, in the sense of abode for animals, is a particular 

 combination of certain environmental conditions, physical and 

 vegetational, and to some extent, animal, uniform over a certain 

 area. More or less variability within this area is the rule, and 

 we may consider the area of the habitat as larger or smaller, 

 according to the degree of uniformity of conditions. In these 

 areas of varying degree, plant and animal communities of 

 various degree find their existence. Ecological classification, 

 aside from its dealings with plant and animal individuals, has 

 to do with the recognition and classification of these degrees of 

 likeness and difference of environments and of plant and animal 

 communities. (See Shelford, 'i2b: 355.) The habitat con- 

 sidered most convenient in the treatment of local distribution of 



