'Bird systematics is no longer merely the study of 

 specimens in the museum." Ernst Mayr (1942) 



18 

 Tradition and Racial Isolation 



On every hand is evidence that groups of ani- 

 mals isolated from each other will vary. Poultry, cattle, and hunting dogs 

 have been divided into their many domestic varieties by the artificial iso- 

 lation of selected breeding stock. In the same way, many kinds of barnyard 

 ducks have stemmed from the wild Mallard. Similarly, over a vastly greater 

 period of time, natural isolation has divided many species of wild animals 

 into distinct races; and it was this that gave Darwin his inspiration con- 

 cerning the pattern of evolution. Before Darwin a species was generally 

 believed to be a distinct creation, with the specific characters showing no 

 variation from a certain type. As specimens from many regions were exam- 

 ined, however, it was found that there often occurred differences in size 

 and color, and this discovery led to the description of geographical races or 

 subspecies. The Song Sparrow is a good example of this regional variation; 

 the A.O.U. Committee on Classification and Nomenclature now recognizes 

 thirty-one subspecies for North America (F. C. Lincoln, letter), ranging 

 from the large, dark Aleutian Song Sparrow of the Alaskan islands to the 

 small, pale Desert Song Sparrow of the southwestern United States. 



Geographical isolation is the most widely recognized and probably the 

 most important factor separating different units of population. Ford (1949: 

 92 ) reminds us that geographic isolation is established not only by "barriers 

 such as water or land (to terrestrial or aquatic forms respectively), or those 

 due to altitude, but that mere distance will prevent a free interchange of 

 hereditary material between individuals at opposite extremes even of a 

 continuous range." Mayr (1942:106) defines the subspecies as a "geograph- 

 ically localized subdivision of the species, which differs genetically and 

 taxonomically from other subdivisions of the species." 



In many birds, such as the Downy Woodpecker, Song Sparrow, Robin, 

 and Yellow Warbler, to name only a few, there are numerous races which 



237 



