254 CALIFORNIA ACADEMY OF SCIENCES. 



Dryobates scalaris and D. stricklandi. 

 Dryobaies scalaris bairdi and D. arizonce. 

 Dryobates villosus leucomelas and D. pubescens. 

 Dryobates villosus harrisii and D. pubescens gairdnerii. 

 Tyrannus verticalis and T. vociferans. 

 Aphelocoma sumichrasti and A. sieberii. 

 Agelaius gubernator, A. phozniceus and A. tricolor. 

 Carpodacus purpureus californicus and C. cassini. 

 Calcarius lapponicus and C. pictus. 

 Zonotrichia leucophrys and Z. leucophrys gambeM. 

 Spizella pallida and S. breweri. 

 Junco caniceps and J. cinereus dorsalis. 

 Junco caniceps and J". annectens. 



Amphispiza bilineata, A. belli and A. belli nevadensis. 

 Peuccea mexicana and P. cassini. 



Passerclla iliaca unalasclicensis and P. iliaca megarhy- 

 ncha. 



Pipilo fuscus mesoleucus and P. aberti. 

 Megascopsflammeolus,M. asio maccallii and M. trichopsis. 

 Thryothorus ludovicianus and T. bewickii. 



Plate XII shows the distribution of two of these over- 

 lapping groups. The different races of the great-horned 

 owl (Bubo virginianus) follow the usual lines of climatic 

 modification normal in the eastern states, grayer in 

 the west, very dark on the northwest coast, and almost 

 white in the Arctic regions. Bubo mexicanus, a tropical 

 offshoot of the genus, is marked with black stripes 

 instead of dusky bars. It is quite evident that these two 

 species were developed in separate districts and after- 

 wards encroached to a slight degree upon each other's ter- 

 ritory. It may be well to point here to an exemplification 

 of the inefficiency of climatic influences as the sole cause 

 of a modification. The ladder-backed group of Dryo- 

 bates are southern forms of the same stock as the striped 

 species. In the genus Bubo, on the other hand, the 



