390 PHYSICAL CONDITIONS IN GENESIS OF SPECIES. 



extends over the whole anterior half of the body. Yet the intergra- 

 dation between the two has been so fully traced that these diverse 

 forms art' now held by competent authorities as merely local races of 

 a single species. 



Another case of the increase of red over the same region is aiforded 

 b}^ the golden-winged and red-shafted woodpeckers (genus Colaptes)^ 

 in which yellow in the eastern form is replaced by red in the other; 

 in the middle region of the continent the species being largely repre- 

 sented by individuals in which are variously combined the special 

 characteristics of the two forms. In the present case the black cheek 

 patch of the eastern form is replaced by a red one in the western. 

 Traces of the characteristics of the Avestern type occasionally appear 

 in the most eastern representatives of the eastern type, and, con- 

 versely, features of the eastern bird appear in the most western rep- 

 resentatives of the western, showing at least their close affinity and 

 probable connnunity of origin. 



The Pacific coast region from California northward is character- 

 ized l)y a great accession of color, all the continental species being 

 here represented by forms much darker even than on the Atlantic 

 coast. Here the coloration is duller than at the southward, though 

 perhaps equally strong, the general tendency being to fuscous or 

 dusky tints. We consequently find among the mammals and birds of 

 the United States three strongly marked phases of color differentia- 

 tion among representatives of the same species, characterizing the 

 three most strongly marked climatal regions — a bright, strongly col- 

 ored form east of the Great Plains, a pallid form over the dry 

 central region, and a deeply colored fuscous form over the rainy, 

 heavily wooded region of the northwest coast. Examples of this 

 differentiation are afforded by apparently all the species whose habi- 

 tats extend entirely across the continent, the several local forms being 

 in some species only more strongly marked than in others. Among 

 mammals illustrations are afforded by different species of squirrels, 

 hares, mice, lynxes, deer, etc., and among birds by six or eight 

 species of sparrows, a mnnber of woodpeckers, several flycatchers, 

 thrushes, and w^arblers, the meadow lark, various hawks, owls, etc. 

 Generally these several geographical forms were originally described 

 as dstinct species, and many of them are still thought worthy of 

 recognition by varietal names. As intermediate links began to be 

 discovered they were at first looked upon as the result of hybrid- 

 ity between the supposed distinct species whose characters the}^ 

 respectively combined, but eventually such links were found to be 

 too frequent and too general over the areas where the habitats 

 of the several forms come together to render such a supposition 

 longer tenable, it finally appearing evident that they were only 

 the connecting forms between merely local races or incipient species. 



