388 BULLETIN OF THE 



influence on the. distribution of the terrestrial vertebrates. The distri- 

 bution of the fishes, the aquatic reptiles and certain groups of batrachians 

 is, however, in great measure determined by the hydrographic basins. 

 Hence we meet with relatively more restricted forms among the latter, 

 as well as in insects, moll usks,. and plants, than we find in either mam- 

 mals or birds, the latter class being the most independent of all animals 

 of geographical barriers. 



It has been remarked that the great extent of the Eastern Province, 

 as compared with the Western, is due to the great extent of the low- 

 lands of Eastern North America, or of that area which has an elevation 

 not exceeding eight hundred feet above the sea.* This is unquestion- 

 ably the true reason, there being no highlands of sufficient altitude to 

 interpose serious obstacles to the range of species. Some portions of 

 this area, however, as the Arctic lowlands, do not belong to this region, 

 while large portions of the country included in the Eastern Province 

 more or less exceed that altitude. These differences of elevation are 

 sufficient to cause the marked interdigitation of the fauna? of contiguous 

 regions lying under the same parallels, as in the Eastern United States, 

 where the upper portions of the Appalachian system support a Cana- 

 dian or subalpine fauna and flora as far south as Georgia. Yet this 

 elevation, in consequence of its nearly meridional trend and its lack of 

 perfect continuity, forms a barrier to but few vertebrates except the strict- 

 ly aquatic ones. If, however, the trend of the Appalachian range had 

 been an easterly and westerly one, the influence of these highlands as a 

 geographical barrier would have been most marked. Without the dif- 

 ferences in altitude it affords, the faunas and floras of Eastern North 



forest skirting the streams. At the eastern limit of the prairies, in fact, the distinctively 

 western species begin to appear, thence westward few additional western species being 

 met. with till the edge of the great central plateau of the continent is reached, where 

 the differentiation is further increased not only by the addition of many new forms, hut 

 by the gradual disappearance of eastern types. Whether the addition of a few prairie 

 species be sufficient reason for recognizing a western subdivision of each of the fauna? 

 of the Eastern Province may perhaps be thought questionable. 



The forested portion of the Eastern Province also presents a lack of total uniformity 

 between its eastern and western portions, a few species of birds occurring east of the 

 Appalachians in the Eastern States only a- stragglers, whilst they are quite common 

 west of these highlands. About half a dozen species avoid the region circumscribed 

 by the valleys of the St. Lawrence, Lake Champlain, the Hudson River, and the 

 Atlantic Coast, that are found west of this area. 



* See Baird, Am. Journ. of Science and Arts, 2d Series, Vol. XLI, p. 86. 



