Allen.] ys bos [June 19, | 
GEOGRAPHICAL VARIATION IN NortTH AMERICAN Birps. 
By J. A. ALLEN. 
Probably the birds of no equal area of the earth’s surface are bet- 
_ ter known than those of North America north of Mexico, or of the 
=< 
whole continent southward even to the Isthmus of Panama. No 
museums in the world, probably, possess so large suites of specimens 
of single species as there are of North Ameruae birds in the 
Museum of the Smithsonian Institution and in the Museum of Com- 
parative Zoology, nor from so many localities. In many instances 
single species are represented by hundreds of specimens collected at | 
frequent intervals throughout their known range. Those contained in 
the Smithsonian Institution have been most carefully elaborated by | 
Prof.’ Baird and others, whose reports upon them have justly ac- | 
quired a world-wide reputation for ‘their thoroughness and accuracy. 
Those in the Museum of Comparative Zoology have also been care- 
fully studied. 
Briefly, then, what are the facts and the general results that have 
followed the investigation of this exceptionally large amount of ma- — 
terial? What are the allowable inferences, and what general prin- 
ciples have been apparently established? ‘To answer these ques- 
tions as briefly as may be is the object of the present remarks, — 
premising, however, that the formerly current opinions respecting the 
rank of a certain class of forms heretofore generally regarded as spe- 
cific have been. radically modified. Intergradation has been fre- 
quently traced between widely different forms, a gradual coales- 
cence in scores of instances having been positively established, and 
rendered extremely probable in a large number of others. 
In North America geographical variation exhibits two marked 
phases :— (1) a differentiation with differences of latitude and ele- 
vation, and (2) differentiation with differences of longitude; which, 
for convenience, may be termed respectively latitudinal and longi- 
tudinal variation.t In respect to both, differentiation occurs in dif 
ferent degrees in different groups, in accordance with their general 
tendency to variation, or, as it were, in proportion to their normal de- 
gree of plasticity. In regard to variation with latitude the modifica- 
tions are apparently more general than in what I have termed longi- 
tudinal variation. In latitudinal variation the differentiation affects 
not merely color, but size and the details of structural parts, whereas 
1 See Bull. Mus. Comp. Zool., Vol. 11, pp. 229-247, et seq., April, 1871. 
