Vol. X 

 '893 



Allen, Origin and Distribution of N. A. Birds. 1*7 



respectively to the eastern or western sides of the continent, have 

 originated in some part of their present habitats, at a somewhat 

 remote period, but most of them doubtless since the retreat of 

 the glacial ice. In earlier times the barrier separating the 

 ranges of species occupying respectively the eastern and western 

 parts of the United States must have been much stronger than it 

 is at present ; and it will be less doubtless a few decades hence 

 than it is now. There is good evidence that the eastern 

 species are gradually extending their range westward, as I have 

 already pointed out in the case of Colaptes auratus, and that 

 western species are, in some cases at least, extending their range 

 eastward. This is doubtless in great part due to, and is certainly 

 aided by, the westward extension of agriculture, which is so 

 rapidly transforming regions not long ago thought to be almost 

 irreclaimable deserts. Irrigation and tree-planting, and the 

 general pursuits of agriculture, cannot fail to extend westward 

 the ranges of many eastern species, as the Bobwhite and various 

 Sparrows, and probably of many of the smaller Passeres. 



Part II. — The Faunal Subdivisions of North America, 



CONSIDERED WITH REFERENCE TO THEIR RELATION- 

 SHIPS, Classification and Nomenclature. 1 



1 . — Faunal Areas. 



In zoogeography it is necessary to recognize faunal areas 

 varying in grade and importance, just as in zoology it is 

 necessary to divide animals into groups differing in rank, as 

 classes, orders, families, genera and species. The terms 

 employed in zoogeography, however, have not been used with 

 the same precision as the practically corresponding terms in 

 zoology. Identical designations have sometimes been used in 



1 The classification and nomenclature here adopted is the same as in my recent 

 paper on 'The Geographical Distribution of North American Mammals' (Bull. Am. 

 Mus. Nat. Hist., Vol. IV, pp. 199-243, Dec. 1892). Also much of the matter, as 

 regards the generalities of the subject, is the same in substance and often in phrase- 

 ology. The accompanying maps are also reproduced, by permission, from the paper 

 just cited. As that paper is not likely to be readily accessible to the majority of the 

 readers of 'The Auk,' it is hoped that the reproduction of some matter previously 

 published in another connection may not be considered reprehensible. 



