ELEVENTH ANNUAL REPORT. 167 



genera of owls. Divided among these are twenty species and 

 twenty-two sub-species, or geographical races, making in all forty- 

 two different forms. These genera are as follows, with the number 

 of forms in parentheses : 



A. SUB-COSMOPOLITAN. 



Sfrix — Barn Owls (i). Otits — Screech Owls (13). 



Asia — Long- and Short-eared Btibo — Great Homed Owls (6). 



Owls (2). Glaucidium — Pygmy Owls (4). 

 Syniiutii — Barred Owls (5). 



B. CIRCUMPOLAR. 



Scotiaptcx — Great Gray Owls (2). A'yctea — Snowy Owls (i). 

 Cryptoglaux — Saw-Whet Owls (3). Surnia — Hawk Owls (2). 



C. CONFINED TO THE NEW WORLD. 



Speotyto — Burrowing Owls (2). Micropallas — Elf Owls (i). 



In tracing the distribution of these dozen groups we find that 

 they fall naturally into the three classes A, B, and C. 



Owls have become so well adapted to their peculiar mode of life, 

 and are so well protected from strenuous competition with other 

 forms, that they have changed but little throughout, perhaps mil- 

 lions of years. In the geological periods of the Oligocene and Mio- 

 cene, a temperate climate existed as far north as Alaska and Green- 

 land, and this, wath the land bridge which joined North America 

 to Asia, explains the large proportion of cosmopolitan genera of 

 owls. Class A. If we may judge by the distribution of insects 

 and mammals, the trend of diffusion across this northern con- 

 tinental isthmus was chiefly from Asia into North America and 

 rarely in the reverse direction. In a strict geographic sense the 

 continental mass of North America is still connected with Asia, 

 as the continental shelf unites the two. and a rise of the sea 

 bottom of only two hundred feet would result in a dry land bridge 

 between the Old and the New World. We can but theorize as to 

 the early distribution of these owls, it being impossible without 

 palsontological evidence, to indicate the center of origin of genera 

 which are cosmopolitan. 



The Circumpolar species, Class B, also doubtless inhabited 

 North America during these early periods. Later, in the Pleis- 

 tocene, all were forced southward bv the onward march of the 



