GEOGRAPHICAL DISTRIBUTION. 17 



II. GEOGRAPHICAL DISTRIBUTION. 



1 have already in the " Bulletin of the Museum of Comparative 

 Zoology" at Cambridge 1 given as correct an idea of the distribution of 

 our species as was possible. In repeating it here (with such additions 

 and corrections as have been suggested by subsequent investigations), 

 I cannot too strongly urge, in extenuation of its imperfection, the mea- 

 greness of the data on which some of my views are founded. I may say 

 with exact truth that California, New England, and the States north of 

 the Ohio River are the only ones which have been thoroughly searched. 

 The species of the rest of the country are known only by the researches 

 of few and widely separated resident naturalists, from the collectors 

 sent by my father, and by collections made by my correspondents while 

 travelling in various sections of the country. The last sources of in- 

 formation are restricted to purely accidental localities. There has been 

 no systematic investigation of vast tracts of intervening country or of 

 some very important points. 



The subject must be studied in connection with the chapter on the 

 same subject in Vol. I. (p. 99). I need not add that from the proper 

 sources the student of distribution must have a thorough knowledge of 

 the physical geography of North America. 



The limits of the fauna at the South correspond quite accurately 

 with the political limits of the United States. The Mexican fauna has 

 lately been investigated by Messrs. Fischer and Crosse in the exhaustive 

 work on " Les Mollusques Terrestres et Fluviatiles du Mexique et de 

 l'Amerique Centrale." The northern limit of the fauna is formed by 

 climate alone. Thus our limits comprise all the continent of North 

 America, from the extreme north to San Diego and the Rio Grande. 



Properly speaking, there are two distinct faunas within these limits, 

 the Pacific and Eastern, with perhaps a third in the Central Basin, but 

 for convenience they are all treated as part of the North American 

 fauna. I have therefore designated these as 



I. The Pacific Province. 



II. The Central Province. 



III. The Eastern Province. 2 



i Vol. III. No. 9. 1873. 



2 In the work of Wallace quoted below, North America is designated as the Nearctic 

 Region. The subdivisions proposed by him correspond almost exactly with my own 



