LAWS OF VAEIATION. 195 



secondly, differentiation with differences of longitude, wliicli, 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 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 the Government reports on the exploration and survey of the various 

 Pacific Railroad rentes. 



" Latitudinal variation presents the following phenomena, which are of such general 

 occurrence that even the exceptions, if such there really be, are exceedingly few : 



" Ist. 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. 



"2d. 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, thi^k, 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 greatel 

 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 macked and less 

 general than the increase of the bill, with which it evidently correlates. 



"4th. 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. 



"5th. In respect to 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 (6) 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 lees 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 



