386 PHYSICAL CONDITIONS IN GENESIS OF SPECIES. 



creases in intensity southward till in Louisiana it becomes deep red- 

 dish orange, the dorsal surface also becoming- at the same time some- 

 what darker. Excepting the fox squirrels and a Pacific-coast variety 

 of the chickaree, all the squirrels living north of Mexico have the 

 lower parts white, while those inhabiting tropical America have the 

 lower parts fulvous, dee}) "golden, orange, or even dark brownish 

 red, specimens with the belly white being excej^tional, though occa- 

 sionalh^ occurring in several of the species. 



Mammals tend strongly to run into melanitic phases, which are 

 especially developed at particular localities or over limited regions, 

 but whether or not the result of geographical influences is not clearly 

 evident. The whitening of the pelage in winter at the north in a 

 considerable number of species of mammals and in one genus of birds, 

 and not elsewhere, is, on the contrary, a strictly geographical phe- 

 nomenon, but seems to be the result of other than the ordinary causes 

 of geographical variation in color. Its occurrence in some species 

 and its absence in others closely allied to them is a fact not readily 

 explained. It shows, however, how differently different animals are 

 affected by the same influences. The change to a white winter livery 

 is more complete in the higher latitudes, where the whiteness per- 

 vades the pelage to a greater depth and continues for a longer period, 

 the change being only partial in the southern representatives of 

 species that exhibit this seasonal change of color. 



In respect to southward increase in color among birds, a few ex- 

 amples only out of the many almost equally striking can be here 

 given. These will be chosen from widely different groups and will 

 represent localities remotely separated, as well as very diverse styles 

 of coloration. In comparing, for instance,. New England examples 

 of the common quail Avith others from southern Florida the colors 

 are found to be so much stronger and darker in the southern birds 

 as to give the aj^pearance of their being entirely distinct species, 

 particularly when the smaller size and larger bills of the southern 

 race are also considered. While in the northern birds the color of 

 the dorsal surface is gray and rufous, slightly varied with black, the 

 gray is wholly wanting in the the southern type, the rufous is much 

 stronger, and the black markings are very much broader. The lower 

 surface is varied by transverse bars of black and white, but while 

 in the northern birds the white bars are twice, or more than twice, 

 the width of the l)lack ones, in the southern birds they are often of 

 equal width ; or the black bars may be the l)roader, with much more 

 black bordering the white throat patch, giving, on the whole, a very 

 much darker aspect to this region of the body. Yet when a series 

 is brought together from many intermediate localities, there is found 

 to be a complete intergradation between the most extreme phases. 

 In the common towhee the style of coloration is entirelv different 



