1872.) 2402 - [Allen. 
color appears to be the main element affected by longitudinal varia- 
tion. The fact of variation in size has been conceded asa general 
law by the majority of at least American ornithologists and mam- 
malogists since it was so fully established by Prof. 8. 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 explorations and surveys of the various Pacific Railroad Routes. 
Prof. Baird then and subsequently} called attention to the fact of the. 
ereater length of the tail in several species of birds at certain local- 
ities, 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 peninsular 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 differences 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, apparently unconnected by inteyr- 
mediate forms. Subsequently, however, it has been found that they 
are not trenchantly separated, intermediate forms so linking them to- 
gether that they can be only vaguely diagnosed. These connecting 
links, inhabiting — at least in the breeding season — localities inter- 
mediate in geographical position and in climatic conditions to those 
frequented by the more extreme forms, suggest an intimate genetic 
relationship and a differentiation mainly or wholly through climatic 
influence, or the diverse conditions of environment. 
Latitudinal variation presents the following phenomena, which are 
of such general occurrence that even the exceptions, if such there 
really be, are exceedingly few. 
1. As regards Size. ‘There is a general reduction in the size of 
the individual from the north southward, amounting not unfrequently 
to as hich as ten to fifteen per cent. of the maximum size of the 
species. The reduction is much greater in some species, and in some 
‘groups of species, than in others, but is almost invariably consider- 
able and easily recognizable. 
2. In respect to the Bill. The variation of the bill is somewhat 
‘inverse to that of the general size, as a rule the southern forms hav- 
|\ing generally relatively, and often absolutely, larger bills than north- 
1 Amer. Journ. Sci. and Arts, Vol. x11, 1866. 
