No. 610] MUTATION THEORY AND SPECIES-CONCEPT 587 



species of Otus Asio Stephens (formerly known as Meg- 

 ascops asio Kaup) over the North American continent. 

 While such a map is onlv approximately accurate, it 

 shows that in general only one subspecies occupies a given 

 geographic areaJ There are, however, certain excep- 

 tions. Thus in Central Colorado Otus asio aikeni and 

 Otus asio maxivellice both occur, the former finding here 

 its northern limit from Texas and Mexico, the latter the 

 southern limit of its range from Montana. It is stated, 

 however,^ that they occur in Colorado chiefly at different 

 altitudes, maxwellim up to 6,000 ft. and aikeni from 5,000 

 to 9,000 ft. This is the reverse of what might be expected, 

 since aikeni is the more southern form. But Mr. Aiken 

 states^ that at Colorado Springs maxwellice occurs only 

 in winter and aikeni only in summer, indicating a slight 

 migration. Again, gilmani and cineraceus—the latter 

 somewhat darker with coarser pencilings and averaging 

 slightly larger in size— both occur in southwestern Ari- 

 zona, but, according to Swarth,^*^ although both birds may 

 occasionally be taken in the same locality, this is only in 

 winter when cineraceus comes down from the higher alti- 

 tudes to the different life zone of the hot Lower Sonoran 

 valleys occupied by gilmani. 



The differences between these various subspecies are 

 chiefly in density of coloration and in size.^^ Thus Otus 

 asio ncBvius is larger than Otus asio asio and is also lighter 

 in coloration, with more white on the under parts. The 

 subspecies mccallii in Texas and northern Mexico is in- 

 termediate between these in size, but is paler than either 



7 It may be pointed out that there is sometimes discernible a tendency for 

 systcmatists to call a form a subspecies or a species according to whether or 

 not it is the only form in a given area, thus making the geographical rela- 



8 Cooke, W. W., 1897, "The Birds of Colorado," Bull. No. 37, Agric. 

 Expt. Sta., Fort Collins, Colo., p. 7S. 



No. 44, Agric. Expt. Station, Fort Collins, Colo., p. 160. 



loSwarth, H. S., 1916, "The Sahuaro Screech Owl as a Recognizabl* 

 Race," Condor, IS: 163-165. 



mens in the Museum of Vertebrate Zoology of the University of California. 



