250 CALIFORNIA ACADEMY OF SCIENCES. 



The screech owl ( Mpgascops asio) is a form of especial 

 interest, from the fact that it is intermediate between 

 such groups as the song-sparrows (Melospiza) and horned 

 larks (Otocoris), in which there is but one species in the 

 east and west, and such as the blue jays (Cyanocitta) in 

 which the Rocky Mountains divide the group into two 

 species. In the Megascops asio group there is but the one 

 species, but east of the Rocky Mountains it is dichro- 

 matic, and west of them monochromatic. If, either 

 from the action of natural selection or from some con- 

 stitutional weakness, the gray phase should die out in 

 the east, it is obvious what very different species would 

 be found on the two sides of the mountains. The fol- 

 lowing table, based on Ridgway's Manual, shows the in- 

 fluence climate has had on the different forms. It is to 

 be used in connection with Plate X. 



1. M. asio, normal. 



2. florhliinuK, darker, markings more nuni- 

 Eastern (dichromatic) { erous and extended. 



.3. marrallii, darker than 1, light mottlings 

 coarser and more conspicuoTis. 



4. trlrhojisis, nearly pure ash gray and black. 

 Bars on lower parts numerous and narrow. 



5. benilirri mostly grayish brown, marks 



•.„ ,„ , , „^, • , J below fewer and coarser than in 4. 



Western (monochromMic) / 



6. l-eiinirotiu, phimage tending toward a deep 

 umber brownish. 



7. mnxtvi'llim, "very light colored, white 

 \ largely predominating on lower parts." 



The towhees (Pipilo), are of peculiar interest in the 

 study of the evolution of color, but at present I will 

 only allude to two species which are influenced by 

 climatic agencies — P. i ryihropJUhuhiMS and P. macidatus. 

 These two species are so very much alike in their gen- 

 eral colors that at first sight they hardly seem separable 

 as distinct species. The females, however, are unlike 

 in color, in the former species being black where the 



