I02 Corresfo7ideuce. [January 



Of course it may be urged that this question has ah-eadv been fully 

 discussed in the writings of Messrs. Baird, Coues, Ridgwaj, Allen, and 

 others ; but some of the readers of -The Auk' have not access to these 

 papers, and a summary of their contents will be very acceptable to those 

 in whose interest the present communication is framed. 



Very respectfully, 

 St. ^yohn. N. B. Montague Chamberlain. 



[Our correspondent's points are well taken, and we will endeavor to 

 briefly explain. First, "Wny was it considered necessai-y to institute that 

 division in zoological classirication termed 'variety' for which trinomials 

 are used V From the context our correspondent seems to imply that this 

 is an innovation peculiar to American ornithology. So far from this 

 being the case, 'varieties' are recognized in all departments of zoology, 

 and also in botan}^ and by all authors of authority the world over, in 

 varying extent, however, in different groups and by different writers. 

 For the forms here referred to as 'varieties,' various tei-ms are in more or 

 less current use, some of which are more explicitly distinctive of what 

 is meant than is the more elastic designation 'variety.' Among such 

 terms may be cited 'subspecies,' 'conspecies,' 'incipient species,' 'imper- 

 fectly segregated species,' 'geographical races,' 'local forms,' etc. These 

 all imply the character of the forms thus designated, namely, that they 

 are intergrading., which, while characterized by differences easily recog- 

 nized in their well-developed phases, yet so coalesce through intermediate 

 stages of differentiation that they run the one into the other and cannot 

 be sharply defined. On the other hand, 'species' are forms that do not, or 

 at least are not known to intergrade, but are separated by a hiatus of 

 o-reater or less extent. Complete separation is therefore the criterion of 

 species, intergradation of subspecies, conspecies, or varieties. "But," our 

 correspondent may ask, "why is it necessary to recognize intergrading 

 forms at all.?" The extent to which they shall be recognized is a matter 

 of judgment, and practice in this regard must ever vary with the predi- 

 lection of the writer, some deeming it advantageous to recognize forms 

 by name that others will regard as not sufficiently differentiated to render 

 their recoo'nition necessary in nomenclature. "Varieties,' or subspecies, 

 are usually geographical, and in manj' cases evidently result from the 

 varying conditions of environment which prevail within the habitat of a 

 species of wide or continental distribution, these varying conditions being- 

 due to diff'erences of latitude, elevation, or topographic features — in 

 other words, to differences of climate, as regards, notablj^, temperature and 

 moisture. For'example, our common Song Sparrow inhabits the greater 

 part of the North American continent, but is represented in different 

 parts of it by quite diverse forms, just as the continent itself embraces 

 wide areas over which prevail climatic conditions very different from those 

 characteristic of other parts. Every one at all conversant with North 

 American birds knows that the Song Sparrow of the States east of the 

 Mississippi River is very different from the Song Sparrow of the great, 



