GEOGRAPHICAL VARIATION IN NORTH AMERICAN BIRDS. 535 
and longitudinal variation.* In respect to both, differentiation 
occurs in different degrees in different groups, in accordance with 
their general tendency to variation, or, as it were, in proportion 
to their normal degree of plasticity. In regard to variation with 
latitude the modifications are apparently more general than in 
what I have termed longitudinal variation. In latitudinal varia- 
tion the differentiation affects not merely color, but size and the 
details of structural parts, whereas color appears to be the main 
element affected by longitudinal variation. The fact of variation 
in size has been conceded as a general law by the majority of at 
least American ornithologists and mammalogists since it was so 
fully established by Prof. S. F. Baird in 1857 and 1858, in his 
admirable reports on the mammals and birds of North America, 
published in the series of Government Reports on the explora- 
tions and surveys of the various Pacific Railroad routes. Prof. 
Baird then and subsequently + called attention to the fact of the 
greater length of the tail in several species of birds at certain 
localities, and cites instances of the larger size of the bill at 
southern points, and the paler color of the plumage of the birds 
of the Plains and the arid peninsula of Lower California. All 
his subsequent works have furnished numerous citations of similar 
variation with locality, but instead of insisting upon any common 
tie connecting these phenomena as the result of general laws, they 
were viewed as evidences of specific differentiation. The differ- 
ences are, indeed, so great between many of the forms now known 
to intergrade that it is not surprising that they were regarded as 
different species when known from only a few examples, appar- 
ently unconnected by intermediate forms. Subsequently, however, 
it been found that they are not trenchantly separated, inter- 
Mediate forms so linking them together that they can be only 
vaguely diagnosed. These connecting links, inhabiting — at 
least in the breeding season — localities intermediate in geograph- 
ical position and in climatic conditions to those frequented by the 
_ More extreme forms, suggest an intimate genetic relationship and 
4 differentiation mainly or wholly through climatic influence, or 
the diverse conditions of environment. 
Latitudinal variation presents the following phenomena, which 
‘re of such general occurrence that even the exceptions, if such 
“ere really be, are exceedingly few. 
* See Bull. Mus. Comp. Zool., vol. ii, pp- 220-247, et seg., April, 1871. 
t Amer. Journ. Sci. and Arts, vol. xli, 1866. 
