186 BULLETIN OF THE 



Part III. 



On Individual and Geographical Variation among Birds, considered 



in respect to its bearing upon the J'alue of certain assumed Specific 



Ch evaders. 



A systematic investigation of the extent and character of individual 

 variation in birds seems not to have hitherto been attempted; in fact, 

 few collections exist that furnish the material necessary to such a 

 work. In occasional instances considerable differences between indi- 

 viduals of the same species, other than those that result from age and 

 sex, have, however, already been pointed out, but these instances seem 

 to have been generally, but improperly, regarded as exceptional cases. 



The collection of birds in the Museum of Comparative Zoology now 

 offers unusual facilities for a general investigation of this subject, most 

 of the common species of Eastern North America being each repre- 

 sented by fifty to one hundred and fifty or more specimens. The 

 greater part of them having been collected in Southern New England, 

 and a large proportion in Eastern Massachusetts, they are the more 

 valuable for this purpose, from their having been collected essentially 

 from the same locality. The examination of this material has disclosed 

 a hitherto unsuspected range of purely individual differentiation in 

 every species thus far studied. At the same time regard has been 

 had to the more obscure seasonal variations in color, and to the gen- 

 eral differences that dfpend upon age, including such as result from 

 senility as well as from immaturity. Local or geographical variations 

 have likewise been carefully considered, with results that a short time 

 since were unsuspected. These several lines of investigation have 

 shown that in many instances what have been regarded as reliable char- 

 acteristics of species have in not a few eases really little or no value; 

 that the importance of many diagnostic features has been too highly 

 estimated, and that consequently a careful revision of our published 

 fauna' will be necessary for the elimination of the merely nominal 

 species. In the following pages many of the data which have led to 

 these conclusions will be presented. 



Individual variation not only affects color and size, but the propor- 

 tions of different part-, as the relative size ami form of the wings, tail, 

 bill, toes, and tarsi, including the skeleton as well as the external organs; 



