LAWS OF VARIATION. 195 
secondly, differentiation with differences of longitude, which, for convenience, may be 
termed respectively latitudinal and longitudinal variation. In respect to both, differ- 
entiation occurs in different degrees in different groups, in accordance with their gen- 
eral tendency to variation, or, as it were, in proportion to their normal degree of plas- 
ticity. In regard to variation with latitude, the modifications are apparently more 
general than in what I have termed longitudinal variation: the differentiation aifects 
not merely color but size and the details cf 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 the Government reports on the exploration and survey of the various 
Pacific Railroad routes. 
‘¢ Latitudinal variation presents the following phenomena, which are of such general 
occurrence that even the exceptions, if such there really be, are exceedingly few: 
“1st. As regards size: There is a general reduction in the size of the individual, 
from the north southward, amounting not unfreqaently to as high as 10 to 15 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 considerable and 
easily recognizable. 
‘62d. 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 having generally relatively, and often 
absolutely, larger bills than northern ones, the increased size taking different propor- 
tions in different species and different styles of bill. Those of a stout, thick, conical 
form generally increase in size, but especially in thickness. Those of a slender or terete 
form become slenderer and relatively longer at the southward, with a decidedly greater 
tendency to curvature. 
‘3d. In respect to the claws: A similar increase in size is apparent in the claws, 
especially in that of the hallux, at southern localities, perhaps less marked and less 
general than the increase of the bill, with which it evidently correlates. 
“Ath. In respect to the tail: A marked elongation of the tail at the southward has 
been noticed in many cases, both in Cape St. Lucas birds (Baird) and in those of Florida. 
“‘Sth. Jn respect te color: The differences in color are especially obvious, and may 
be reduced to two phases of modification: (a) a general increase in intensity at the 
southward; and (b) an increase in the extent of dusky or black markings at the expense 
of the intervening lighter or white ones, or, conversely, the reduction in size of white 
spots and bars. Under the general increase in intensity, the iridescence of lustrous 
species becomes greater, and fuscous, plumbeous, rufous, yellow, and olivaceous tints 
are heightened in species with the color continuous in masses. 
‘* Under the repression of light colors, the white or yellowish edgings and spots on the 
wings and tail become more or less reduced ; and frequently, to a great degree in spe- 
cies barred transversely with light and dark colors, the dark bars widen at the south- 
ward at the expense of the white or lighter ones, sometimes to such an extent as to 
greatly change the general aspect of the species, as is the case in the Ortyx virginianus 
of the Atlantic States, and in other well known species; also, under the tendency to 
the increase of dark colors, longitudinal streaks and blotches in a light ground increase 
in extent and intensity of color.’ 
Prof. Allen, after mentioning the coincidence of pale colors with dry 
