1920] Swarth: Revision of Avian Genus Passerella 91 



of variation of Passerella is toward darker coloration as this region 

 is approached from the northward, it is doubly surprising to find the 

 fox sparrows upon these islands developing no departure of this sort 

 from the mode of the subspecies farther north. On Vancouver Island 

 is the abruptly darker fuligiTiosa, but this race occurs upon the main- 

 land also, in northwestern Washington, so insularity here cannot be 

 brought forward as the cause of the difference. 



Thus in the Unalaschcensis group, while there is continuous, 

 gradual, and well defined variation in characters from one extreme 

 to another, with abrupt accentuation of certain features at intervals 

 (serving for the definition of the several subspecies), the dividing lines 

 between the forms do not, to our knowledge, fall where there are 

 physical barriers to distribution (save in insidaris), and we do know 

 that there are many such barriers that have no effect. In just one 

 case, in the subspecies insidaris, is there a race restricted to an island 

 habitat. 



Now, turning to the Schistacea group of subspecies, we find almost 

 exactly the same sort of variation under widely different conditions. 

 These birds inliabit mountain tops; they form colonies occupying 

 Boreal "islands," widely separated each from the other by broad 

 expanses of intervening territory, usually' of a lower zone. Through 

 these subspecies variation can be traced that is just as gradual and 

 just as continuous as in the Unalaschcensis group. Briefly, in the 

 Schistacea group there are two lines of variants, one a series of rela- 

 tively brownish colored forms, the other, of grajash colored ones. In 

 the first line, beginning with the subspecies schistacea — a brown- 

 colored, small-billed race with tail of medium length — a series of steps 

 may be traced through geographically adjacent forms, culminating 

 in the similarly brown-colored, but enormously large-billed and short- 

 tailed subspecies, hrevicauda. The second series of variants begins 

 with canescens — color gray, bill small, tail of medium length — and 

 ends in the gray-colored, huge-billed and long-tailed stibspecies, 

 stephensi. 



Each subspecies of the Schistacea group appears to be circum- 

 scribed in its distribution by physical limitations to territory suitable 

 to fox sparrows ; the different races are separated by the intervention 

 of unsuitable life zones. Specimens occur showing intergradation 

 between the forms, but, apparently, in no case is there continuous 

 distribution such as seems to obtain on the northwest coast. The 

 point to be emphasized here, however, is that while the different sub- 



