1920] 



Swarth: Revision of Avian Genus Passerella 



101 



was from north to south. The north and south lines of development, 

 approximately parallel, seen in the distribution of the subspecies of 

 the Schistacea group may have resulted from a fan-like dispersal from 

 a common center along favorable lines on the Sierra Nevada and 

 adjacent moimtain ranges. That there is an apparently comparable 

 chain of variants stretching at right angles, from east to west, may 

 be entirely fortuitous, the several links being really unconnected, 

 genetically, but merely representing comparable degrees of evolution 

 in the two north and south lines. Similarly, local dispersals from 

 points along these lines may enter into the problem. 



o J 



,-^ 



~^'^ 



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Fig. F. Bills of four subspecies of fox sparrow of the Schistacea group, show- 

 ing the trend of variation in this member in the four contiguous races extending 

 from northern Nevada (schistacea), through northeastern California (fulva) and 

 the Mount Shasta region (m-ariposae) to the inner coast ranges of northern Cali- 

 fornia (brevicauda) ; natural size. 



o. Passerella iliaca schistacea, adult male; no. 9054; Musi Vert. Zool. ; Pine 

 Forest Mountains, Nevada; June 19, 1909. 



b. Passerella iliaca fulva, adult male; no. 14800, Mus. Vert. Zool.; Sugar Hill, 

 Modoc County, California; May 30, 1910. 



c. Passerella iliaca irmriposae, adult male; no. 29089, Mus. Vert. Zool.; Sissou, 

 Siskiyou County, California; May 15, 1918. 



d. Passerella iliaca brevicauda, adult male; no. 23922, Mus. Vert. Zool.; South 

 Yolla Bolly Mountain, Tehama County, California; August 1, 1913. 



Distribution and Migration 



Differentiation of the Iliaca, Unalaschcensis and Schistacea groups 

 may be assumed to date back to the period when the first two retired 

 northward in the wake of the retreating ice fields at the close of 

 glacial times, and the last became stranded upon the mountain tops 

 of the southland. While differentiation into the three general types 

 may be admitted as having conceivably arisen in this way, it would 

 seem that the variation within each group must be due to some other 

 cause or causes. It is as though we were contemplating three species 

 (the groups Iliaca, Unalaschcensis and Schistacea), each group owing 



