MUSEUM OF COMPARATIVE ZOOLOGY. 389 



America would have extended in regular and parallel zones from the 

 Atlantic seaboard to the central plateau of the continent, whereas they 

 now sweep far southward near the coast, and passing around the Appa- 

 lachian highlands extend northward again along their western base. 



In attempting to determine the number and limits of the ornitho- 

 logical faunoe of a large area, it is evident that the distribution of the 

 birds in the breeding season should be taken as the basis for the inves- 

 tigation rather than their entire range, since during no other portion of 

 the year can the migratory species be regarded as being at their true 

 homes. The species numerously represented are also of far greater 

 importance than those having but few representatives, as it is the 

 common species which are not only the most characteristic, but those 

 whose distribution is at present best known. 



Applying the term fauna, when used in a special sense, to the smallest 



of the natural divisions in zoological geography, and considering faunas 



to be characterized by their general facies, as determined by a peculiar 



assemblage of species, rather than by the restriction of a certain 



number of specific forms within their areas, Eastern North America 



may be considered as embracing seven ornithological fauna?, which 



occupy narrow, somewhat parallel zones or belts of varying breadth, 



extending from the Atlantic coast westward to the great middle plateau 



of the continent. The extent of each in an east and west direction is 



generally many times greater than what may be considered as its 



meridional extent. Their breadth, however, is quite unequal, not 



only as compared with each other, but that of the same fauna varies 



greatly at different points. They have their minimum breadth on the 



slopes of the mountains, and attain their maximum breadth on the 



plains. Each species having its own peculiar limits, which may or 



may not coincide with those of other species, it usually happens 



that at somewhat regular intervals, in passing either northward or 



southward from a given point, a greater number of species disappear 



at some points than at others, at which point also other species first 



appear. These divisional lines usually coincide with some marked 



physical change in the general character of the country, more especially 



in respect to its elevation, and form the boundaries between adjoining 



faunas.* These faunal boundaries, as has been before remarked, coin- 



* The first terrace of the Atlantic slope, which marks not only the transition from the 

 tertiary deposits of the coa<=t of the Middle and South Atlantic States to theolder forma- 



