MUSEUM OF COMPARATIVE ZOOLOGY. 237 



(b) Longitudinal Variation. — In comparing the birds of the Atlantic 

 States with specimens specifically identical from the interior of the 

 continent, one is soon struck with the brighter colors of the latter, and 

 especially with a tendency, in many species, to more ferruginous tints, 

 and to melanism in others. In comparing again the birds of the Mis- 

 sissippi valley with those of the Pacific slope, especially that portion 

 north of the fortieth parallel, a similar difference is also noticeable, the 

 extremes of color variation in truly continental species being met with 

 (especially to the northward of this parallel) at the Atlantic seaboard 

 on the one hand, and the Pacific on the other, between which there is a 

 gradual and, with an exception soon to be noticed, a uniform increase in 

 intensity of color to the westward. This tendency to more ferruginous 

 and melanic colors to the westward is especially marked in Falco pere- 

 grinus* Accipiter fuscus, Circus hudsonius, Buteo lineatus, Buteo 

 borealis, Archibuteo lagopus, Hypotriorchis columbarius, Olus vulgaris, 

 and other species of Strigidce, Tetrao canadensis, Bonasa umbellus, 

 Bernicla canadensis, Bernicla brenta, Larus argentalus, Par us atrt- 

 capillus, Carpodacus purpureus, etc., etc. The western representatives 

 of Melospiza melodia, Passerella iliaca, Jlvico hyemalis, Pipilo ery- 

 throphtkalmus, Parus hudsonicus, etc., differ mainly from their Eastern 

 congeners in their more ferruginous or darker colors, according to the 

 species. 



While the general tendency from the East westward is thus to darker 

 or deeper colors in specimens of the same species, and in representative 

 species of the same genus, the rule is not without exceptions, nor is the 

 transition quite uninterrupted. On the arid sterile plains the repre- 

 sentatives of not a few, and probably of most, species are much lighter 

 colored than their relatives either to the eastward or to the westward. 

 Also at the southward on the Pacific slope there is not the tendency to 

 deeper colors seen farther to the northward, specimens from North- 

 western Texas, New Mexico, much of the Colorado basin and Lower 

 California, being lighter than others of the same species from Northern 

 California, Oregon, and Washington, an explanation of which will be 

 suggested later.f 



In comparing again the European representatives of cireumpolar 

 species with their representatives in Eastern North America, a difference 



* For the synonymy and other remarks on these species, see Part IV. 

 f See below, p. 239 et seq. 



