MUSEUM OF COMPARATIVE ZOOLOGY. 245 



Not a few naturalists have hence adopted the test of intergradation, 

 which seems a reasonable and an unobjectionable one. The question of 

 species and of specific synonymy is thus simplified to this: that when- 

 ever two forms which have both received names are found to intergrade, 

 the more recent name shall become a synonyme of the older. Some, 

 however, still urge that every recognizable form, however closely 

 allied to others, and even intergrading, should be recognized by a 

 binomial epithet, and that whether we call them species, or varieties, 

 or races, or simply forms, that such names are none the less convenient 

 expressions for certain facts. It seems to me, however, that there are 

 insuperable objections to this course ; for however distinct the extreme 

 geographical forms of a species may be, a vast proportion of its repre- 

 sentatives are intermediate to them, and could never be but doubt- 

 fully referred to the one rather than to the other. Ordinarily, for 

 instance, in the birds of the Atlantic slope, the representatives of a 

 given species at the extreme north of its breeding range almost always 

 differ very tangibly from its representatives at the extreme southern 

 limit, sometimes more, sometimes less, according to the species. Those 

 living only a little to the northward of the middle region differ less from 

 the extreme southern type than the extreme northern type does, and those 

 a little to the southward of the middle region differ still less from the 

 southern type, and are qviite distinguishable from the extreme northern 

 form. In other words, in species ranging from Southern Labrador or 

 Northern New England to Florida, of which there are numerous un- 

 questioned instances, specimens from Southern New England differ 

 somewhat from the more northern ones ; those from Southern New Eng- 

 land from those of Southern New Jersey and Eastern Maryland, and 

 these latter from those of Georgia and Florida. It hence depends en- 

 tirely upon individual predilection whether two, three, or four " species " 

 or " binomial forms " shall be recognized ; and in either case there is 

 the same difficulty in disposing of the intermediate types. Again, speci- 

 mens from the Mississippi valley differ more or less from their relatives 

 from the Atlantic coast, the central plains, and the Pacific slope. Here 

 again similar difficulties are encountered. Hence it is necessary to 

 decide between recognizing a single binomial form, with a considerable 

 but definite range of climatic variation, or three, or six, or nine, or even 

 more, which cannot be rigidly defined, and between each of which will 

 always be found a greater or less proportion of intermediate types, 



