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BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



third, have a general distribution throughout Temperate North America, 

 there are four genera and one subgenus peculiar to the so-called East- 

 ern Province, five genera and one subgenus mainly restricted to the 

 Middle Province, and five genera and two subgenera almost wholly lim- 

 ited to the geographically much smaller Western Province. In addition 

 to this, there are five other genera and one subgenus common to the 

 greater part of the Middle and Western Provinces that are not found in 

 the Eastern.* The genera that may be regarded as characterizing the 

 middle temperate region of North America and their relative distribution 

 is shown in the subjoined table. 



Terrestrial genera and subgenera of Middle North America (between the mean annuals of 36 



and 68 F.), not found in the Arctic find Cold Temperate latitudes. 



[NOTE. Subgenera are enclosed in parentheses.] 



* Chiefly tropical. 



t Occurs also in Asia. 



Summary. 



Total number of genera (plus 7 subgenera) 38 



Of general distribution 13 



Peculiar to the Eastern Province 5 



Peculiar to the Middle Province 6 



Peculiar to the Western Province 7 



Common to the Western and Middle Provinces, but not found in the Eastern 6 



Mainly tropical or subtropical 8 



* Mr. Wallace, in his late work (Geogr. Dist. Anim., vol. i, p. 6), refers to the Rocky 

 Mountains as forming a barrier to species, " almost all the mammals, birds, and in- 

 sects " belonging to different species on the two sides of the Rocky Mountains. Noth- 

 ing, so far as mammals and birds are concerned (and I am informed by good authori- 

 ties that the same is true of insects), could well be further from the truth. Only in 

 rare instances do the Rocky Mountains form such a barrier, the division between the 

 Eastern and Middle Provinces being more than six hundred miles to the eastward of 

 this range, while the boundary between the Middle and Western Provinces is formed 

 by the Sierra Nevada chain. The same species, as a rule, range over the greater part 

 of the great elevated interior plateau, of which the Rocky Mountains constitute the 

 'axis. So far as the distribution of both birds and mammals is concerned, the presence 

 or absence of forests, and the accompanying diverse climatic conditions, have far more 

 to do with the limitation of habitat than the commonly so-called "Rocky Mountain 

 barrier ". This is obviously due to the longitudinal direction of this supposed barrier, 

 which, if trending in a latitudinal direction, would certainly form an impassable 

 obstacle to very many species. 



