ELEVENTH. ANNUAL REPORT. 181 
GREAT HORNED OWL. 
of environment, with which, in this much diversified region, 
these non-migratory birds have come into contact. 
The plasticity of the owls of this genus is remarkable, con- 
sidering the narrow structural limits within which the differences 
range. About eighty forms have been described from all parts 
of the world. The differences are in the darker or paler plumage, 
and in the greater or lesser size. The large Kennicott Screech 
Owl, which inhabits the coast region from Oregon north to Sitka, 
is dark, sooty brown, streaked with black, and is almost eleven 
inches in length. From this we have gradations, continuous or 
not, according to whether the forms are more or less isolated, 
until we reach the pale, desert-hued Xantus Screech Owl some 
eight inches over all, which is found only in Lower California, 
and because of its isolation has been made a species. The min- 
imum in size, however, is shown by the Dwarf Screech Owl of 
Idaho, a pale form hardly seven inches from head to tail. In addi- 
