No. 571] TERRESTRIAL ASSOCIATIONS 433 



as western species, those for which a number of locality 

 records are available are plainly to be assigned to the 

 prairie province, the range of most of them extending 

 west to the Eocky Mountains, north about as far as 

 Montana, east to Illinois or Indiana, and south to Texas. 



Other animal species bear apparently no relation to 

 province boundaries. Such animals have been discussed 

 by Shelf ord {A: 606, footnote), who shows them to be of 

 three types: (1) Species of scattered but very wide 

 range, covering perhaps several plant realms (animals 

 of local associations of extreme habitats) ; (2) Species 

 occupying only a part of the plant realm in which they 

 belong (animals of such ecological constitution that their 

 range is restricted by some conditions unfavorable in 

 certain parts of the province); (3) Species occupying 

 intermediate ground between two realms — these are few 

 (Ruthven). These exceptional species are found also in 

 plants, so that local associations are occupied by both 

 plants and animals of the scattered-but-wide type of 

 range, while certain subregions, as the Great Plains area 

 of the prairie province, contain associations with both 

 plant and animal species restricted to these less extensive 

 areas. 



Associations of two adjoining provinces may inter- 

 grade, if ecologically similar, or may alternate if dis- 

 similar. Similar associations of two provinces may con- 

 tain the same or closely related species, as with certain 

 grasshoppers which range in both northeastern and west- 

 ern coniferous provinces (D: 173). But these same asso- 

 ciations contain also plant species in common, so that 

 irregularities of range are no greater in animals than in 

 plants. 



2. Distribution Within the Prorinrr: Distribution of 

 Plants and Animals into Communities.— It is seen that 

 plant and animal species may correspond closely in geo- 

 graphic range. There may be also more local corre- 

 spondence in distribution. The plant community has 

 been found by the writer to be the convenient index of the 



