1913] Grinnell-Swarth: Birds and Mammals of San Jacinto 395 



Where adaptable forms continuously through time and space 

 invade radially towards one another, with but a narrow belt of 

 intermediate conditions between the differentiation areas, specific 

 distinction may be attained because of this very narrowness of the 

 intermediate area. The intermediates being non-adapted and 

 therefore relatively useless, would be eliminated. It is thus 

 possible to account for the origin of such now separate forms as 

 Toxostoma redivivum and T. lecontei, and Dryobates scalaris 

 and D. nuttalli. These were doubtless not long ago intergradient 

 forms, therefore subspecies. 



By judicious observation it seems possible to select various 

 degrees in the process of specific separation, on the margins of 

 habitats. Some species are seen to blend perfectly, in others there 

 is broken or mosaic intergradation, in others hybridization, as 

 expressed in cases of fairly constant intervening type, and finally 

 there are the distinct forms with no hybrids in evidence. In 

 such a series . we see the historical sequence in relationship 

 preceding the "full species." 



LITERATURE CITED 



American Ornithologists ' Union Committee. 



1910. Chock-list of North American birds, eel. :'.. revised (New York, 

 American Ornithologists' Union), 4.">i) pp., '1 maps. 



Anthony, A. W. 



1889. New birds from Lower California, Mexico. Proc. Calif. Acad. 

 Sri., (2), 2, 73-82. 

 Baird, Brewer, and Ridgwav. 



1874. A history of North American birds. Land birds. (Boston, 

 Little, Brown, and Company), 2, pp. 1-590, i-vi, 30 pis., many 

 figs, in text. 

 Bangs, O. 



1899. Descriptions of some new mammals from western North 

 America. Proc. New Engl. Zool. Club, 1, 65-72. 

 Beebe, C. W. 



1907. Geographic variation in birds with special reference to the 

 effects of humidity. Zoologica: N. Y. Zool. Soc, 1, 1-41. •"> 

 pis., 1 fig. in text. 

 Brewster, W. 



1889. Descriptions of supposed new birds from western North 

 America and Mexico. Auk, 6, 85—98. 



