THE BIRDS OF THE PRIBILOF ISLANDS. 365 



have been so. Ou the Asiatic side we have no present continuous land connection, 

 but evidently it has been otherwise, for these Siberian species could hardly have 

 learned their way into Alaska over the present bleak and forbidding route. The 

 facts thus require that when the Asiatic or Siberian forms first reached what is now 

 Alaska they did so over continuous or narrowly separated land areas. The American 

 forms only extended their range as the rising or clearing laud became available for 

 their needs. The Siberian species have simply continued their migration over the 

 slowly disappearing land. 



The practical restriction of the family Alcidae to the Pacific, and the actual restric- 

 tion of so many genera and species, would seem to require that when, previous to the 

 glacial periods, ice existed in but small quantities about the North Polar region what 

 is now Bering Straits was tightly closed to the members of this family in the Pacific. 

 If it were not so, it is perhaps impossible to account for the restriction of these birds 

 to the North Pacific, and also for the reason that such Atlantic forms as Plautics, 

 Alca, and Alle failed to diffuse themselves throughout circumpolar areas during pre- 

 glacial times. 



Of the 29 Pacific forms, 16 have close relatives ou the xVtlantic side, but they 

 belong to genera of wide distribution, and, excepting a few laud genera, Passerina, 

 Calcarius, Animodramus, Troglodytes, are water birds of extensive northern habitat 

 and gene.'ic circumpolar distribution, like Bissa, Larus, etc. These last all have 

 extremely well-developed powers of flight. It would seem that where related forms 

 inhabit both sides, the Pacific birds are the larger, with longer and larger bills. Of 

 the various species of the subfamily Fhalerinae nearly all occupy generic or sub- 

 generic divisions by themselves and are consequently distinctly differentiated, few 

 genera containing more than one species and none subspecies. 



The above facts would indicate that the Atlantic members of the Alcidae have 

 been derived from Pacific ancestors during several warm periods of the i^ast, Alle, 

 Plautus, and Alca having been much the earliest.^ Thus, besides the present and 

 past ice, there would seem to have existed a barrier to transpolar mixing and dis- 

 persal of Atlantic and Pacific forms. 



That this ancient Bering Straits land barrier prevented Pacific types from 

 spreading east and west into the Atlantic, and vice versa, seems extremely probable, 

 and that this same barrier may have had considerable to do with the causation of the 

 Glacial epochs seems evident, considering the general topography of the region sur- 

 rounding the Arctic Basin. The result now of damming up the waters flowing south 

 through the Straits, the influence of which is . felt and seen by every traveler in 

 Bering Sea, and even when hundreds of miles south of the Aleutians, would be 

 disastrous in its effects on the i^resent climate of Asia, and especially to North 

 America, and most certainly so to the present distribution of northern forms of life. 

 The North Pacific Basin is a vast amphitheater. The volcanic activities which dot 

 its circumference — the grandest in the whole world, yet now in its last throes — have 

 consumed considerable material that has most evidently been derived from a seaward 

 direction. Destruction and submergence has necessarily followed, and deep water 

 is now found where most probably low, extensive, and volcanic island areas were 



' I consider Alle misplaced wibli the other genera. It is more closely allied with the lower 

 Alcidae, the neossoptiles heing structurally distinct from those of the Alcinae. 



