BIOTOPES AND BIOCOENOSES 141 



different grassland areas — the North American prairies, the pampas of 

 the Argentine, the steppes of Africa, Eurasia, and Australia — are 

 independently populated by members of each faunal area; in spite of 

 the similar general impression the inhabitants are always more nearly 

 allied to those of other biocoenoses in their region than to those of 

 the similar habitats in distant regions. The grassy steppe of Australia 

 is populated by marsupials, the South American by histricomorph 

 rodents, the Holarctic by Muridae. They agree in having strong chew- 

 ing apparatus, a large number of jumping animals equipped with long 

 hind limbs, and a variety of burrowing forms. In addition, numerous 

 necessary differences between the faunae of similar habitats are due 

 to the different degrees of adaptation to the similar conditions. 



Within every more strictly selected biocoenosis the individual spe- 

 cies constantly exhibit differences in their general distribution. 



One important set of animals for the characterization of a biotope 

 are those which are limited to the biocoenosis, the index species, which 

 may be characterized as autochthonous. These are so exactly adapted 

 to the ruling conditions of their habitat that they are unable to live 

 in another environment. They are usually herbivores since the car- 

 nivores and omnivorous animals are characterized by having a wider 

 distribution. Such animals may be distinguished as eucoen, and corre- 

 spondingly in the individual biotopes and biochores: eupolar, in the 

 ice-dominated tundra; eunival, in the snow zone of mountains; eucaval, 

 in caves; euhaline, in the saline inland waters; euvastal, in steppes; 

 eudesertal, in deserts; etc. The number of these most closely adapted 

 index forms is small for most biocoenoses; it increases with the severity 

 of selection, as the conditions depart from the optimum, and is very 

 large, for example, in the biocoenosis of moss, where resistance to 

 long-continued cold, heat, and drought are required, and only such 

 animals can persist as are able periodically to suspend their living 

 functions, awaking to renewed life when favorable conditions reappear. 

 In smaller biocoenoses such autochthonous forms are frequently 

 lacking. 



A much more numerous element in associations is composed of 

 animals which occur also in other biotopes, whether only in similar 

 adjacent ones or in widely scattered very different habitats, as ubiqui- 

 tists (eurytopic forms) . Such members of a biocoenosis may be dis- 

 tinguished as tychocoen. These are frequently adaptable animals, and 

 such forms flourish equally well under very different habitat condi- 

 tions. Tychocoenous animals fall behind the eucoen element in num- 

 bers only when the selection by unfavorable conditions is especially 

 severe. Examples of tychocoen forms are the raven, in the Arctic; 



