l6o POLAR PROBLEMS 



animals had spread to such widely divergent regions as Africa, India, 

 and America. With the relegation of the hypothesis of a polar con- 

 tinent to limbo, the center of origin had to be shifted to some place or 

 places on the continental ring surrounding the deep oceanic Polar 

 Basin. About the same time it became generally accepted that the 

 animals now inhabiting the continent of Europe, northern Asia, and 

 northern America are so closely allied that they are properly con- 

 sidered to form a continuous holarctic zoogeographic province or 

 realm. Obviously, then, the movement, which under the earlier 

 conceptions was thought to have been along radiating north-south 

 meridians, assumed under the new hypothesis an east-west or west- 

 east direction. Even Dr. Jager had realized that the Polar Sea had to 

 have an outlet and argued against a land connection between Eurasia 

 and America across the Bering Strait region, holding that the inter- 

 change of faunas within comparatively recent geological time had been 

 across a broad land uniting Europe to Greenland and the rest of North 

 America. Such also was and still is the contention of various prom- 

 inent zoogeographers. Thus Scharff' assumed this connection to 

 have been by way of Spitsbergen, but, on the present writer'^ having 

 successfully argued against this supposition, Scharff, in his later works, 

 chose the other alternative of a Greenland-Iceland-Scotland land 

 bridge. 



It is not the intention in this article to argue pro or con any of the 

 various hypotheses which have been proposed to explain the present 

 distribution of the holarctic animals or even to mention all of them. 

 Only the more representative views can here be set forth briefly. The 

 hypothesis that some of the constituents of the present boreal faunas 

 have had their origin, and that others have had their secondary center 

 of distribution, in the high lands of the interior of Asia, which con- 

 stitute one of those never submerged continental areas which phys- 

 iographers call "shields," or "bosses," or "Scheitels," has gradually 

 gained almost universal acceptance, and, as a corollary, the opinion 

 that the Bering Strait or Bering Sea land connection has been the 

 principal route by which the exchange of the faunas between Eurasia 

 and America took place. As an illustration of this view I need only 

 cite W. D. Matthew's discussion in his excellent, though obscurely 

 entitled article "Climate and Evolution,"' in which he demonstrates 

 that it is unnecessary, at least for an explanation of the present 

 distribution of the placental mammals, to postulate any other inter- 

 continental land bridge than that of Beringia, as the Bering land 

 bridge has been named by Sushkin. Of course, the hypothesis of the 

 former existence of a land connection in this region is an old conception. 



^ R. F. Scharff: The History of the European Fauna, London, 1899. 



' Leonhard Stejneger: Scharff 's History of the European Fauna [a review], Amer. Naturalist, 

 Vol. 35, 1901, pp. 87-116; reference on pp. 103-104. 



^Annals New York Acad, of Set., Vol. 24, 1914 (pubHshed Feb., 1915), pp. 171-318. 



