MUSEUM OF COMPARATIVE ZOOLOGY. 375 



arated from the B. americana et clangula, and the Collurio excubitoroides 

 from the G. ludoricianus, will serve to indicate the class of so-called 

 species here referred to. 



The Pacific Slope of North America furnishes a similar list of spe- 

 cies, based on either southern or northern forms of others previously 

 known ; and the middle region of the continent its list of similar nominal 

 species, mainly based on the desert forms of widely ranging species. In 

 the northern half of the Old World, also, have the northern and south- 

 ern geographical forms of the same species been specifically separated ; 

 but it is not my intention to call farther attention to them at present. 



As already remarked, the American representatives of circumpolar 

 species differ from the European and Asiatic principally in two ways, 

 namely, in the apparently slightly larger size of the American, and in 

 their somewhat brighter colors ; but specific separations seem to have 

 been based almost as frequently upon some theory of geographical distri- 

 bution, or upon the individual variation of single specimens, as upon the 

 real though slight differences that frequently obtain in such cases. 



PART V. 



On the Geographical Distribution of the Birds of Eastern North Amer- 

 ica, with special reference to the Number and Circumscription of the 

 Ornithological Fauna. 



1. Introductory Remarks. 

 The distribution of plants and animals in circumpolar zones over the 

 earth's surface has been long recognized; Humboldt* first making 

 known the fact of such a natural distribution of the plants, and Agassiz,f 



* Humboldt, A. vox, et Boxpland, Ai.me. " Essai sur la G^ographie des Plantes," 

 etc. 4to. Paris. 1805. 



t Acassiz, Louis. " Essai sur la G(?ographie des Animaux," Pevue Suisse et Chro- 

 nique LitteYaire, Tome VIII, pp. 441-452, 538-585, 1845. •' Note sur la Distribution 

 Geographique des Animaux et de l'Homme," Bulletin de la Societe" des Sciences Natu- 

 relles de Xeuchatel, Tome I, pp. 162-166, 357-361, 366-369, 1845. "Sur la Distribu- 

 tion Geographique actuelle et le mode de l'apparition actuelle des Animaux a la 

 surface du Globe." Ibid., Tom. 2, pp. 347-351, 1847. "Geographical Distribution of 

 Animals," Edinburgh New Philosophical Magazine, Vol. XLVI, pp. 1-25, 1850. 

 Ibid., Christian Examiner, Vol. XL VIII, pp. 184-204, 1850. '-Sketch of tiie Natural 

 Provinces of the Animal World and their Relation to the different Types of Man," Nott 

 and Gliddon's Types of Mankind, pp. lviii- Ixxxii, 1854. Also especially insisted upon 

 in a course of unpublished Lectures delivered before the Lowell Institute, Boston, 

 December, 1869. 



