MUSEUM OF COMPARATIVE ZOOLOGY. 373 



from Florida to Maine, a tendency to a relatively greater elongation of 

 the tail in the Florida specimens has also been noticed, — a feature so 

 well known to characterize a large proportion of the birds of Lower 

 California, as pointed out some years since by Professor Baird, — but 

 this variation is not so frequent as the differences in size, color, and in 

 the length and form of the bill. As already remarked, the tail i3 not 

 usually abbreviated proportionally to the general diminution in size in 

 the southern or Florida forms of the birds of Eastern North America, 

 and in some species it is actually longer than in the larger northern birds. 

 As shown in the above tables, this is the case in Pipilo erythrophthal- 

 mus, Cyanura a-istata, and Colaptes auratus, or in three species out of 

 the seven cited in the last table. 



In numerous instances the southern forms of the birds enumerated 

 in Part IV of this paper have already been specifically separated from 

 their northern relatives ; and if the example of some previous writers 

 was to be followed at least a dozen other similar species might still be 

 added from among the birds of Florida. Some, indeed, might be re- 

 ferred to the already separated West Indian and Mexican or Central 

 American so-called species rather than to the northern type. As al- 

 ready stated, I consider this almost universal similar variation of the 

 southern representatives of species from their northern representatives 

 to be the result of a law of gradual geographical differentiation, and 

 that the interest of science is better subserved by simply recognizing 

 these differences, and the law of geographical variation of which they 

 are the result, than by giving to each newly discovered race a distinctive 

 binomial name ; and the more especially since in numerous instances 

 there is the most indubitable proof of the gradual and almost imper- 

 ceptibly minute intergradation of the extreme northern and extreme 

 southern types, even in cases where they are the most widely diverse. 



In conclusion, it may be stated that the differential diagnoses of the 

 southern types, in cases where they have been specifically separated 

 from the northern, and the comparisons of them made with the northern 

 for the purpose of showing their specific distinctness, are in many 

 cases admirable descriptions of the usual differences found to distinguish 

 the Florida-born birds from their -co-specific representatives born in 

 the Northern States. These differences are commonly solely the fol- 

 lowing : In the southern types the size is smaller, the bill longer, and 

 the colors generally darker ; the latter resulting from the greater pre- 



