G. R. Wieland — Polar Climate in Time. 413 



in passing by either of these routes. At all times since the 

 earliest Mesozoic either animals (or plants) in so doing would 

 so repeatedly have been forced to change their mode of life 

 and endure new conditions during the reproductive season, 

 especially after sharp winters set in in the northern regions, 

 that one must greatly doubt if any ever accomplished the 

 journey after the later Cretaceous. Likewise that any of the 

 synchronous similar faunge or elements of faunse could have 

 come from the south is contrary to the evidence of the fossils 

 themselves, and contrary to what we know of the general north 

 to south movement of animals and plants as so fully and thor- 

 oughly demonstrated by Wallace in his Island Life. Again, 

 that the same series of peculiar genera of horses, dogs, camels, 

 etc., were constantly being evolved independently at widely 

 separated points is prej^osterous. 



Taken singly and collectively, the facts of Mesozoic and Ter- 

 tiary vertebrate distribution in the northern hemisphere explain 

 themselves satisfactorily on no other than the sole remaining 

 hypothesis, namely, that of a common polar origin of the prin- 

 cipal ancestral stocks, which then dispersed secondarily out- 

 wards in waves or impulses from the polar area and spread 

 over America and Eurasia. The various high northern lands 

 mentioned above then become, instead of mere migratory 

 routes, centers of origin and a means of ready dispersion, the 

 various stocks only passing out uniformly from them in the 

 southward direction — that which always offered least resist- 

 ance to migration and extension of habitat. In addition a 

 large part of Northern Siberia maybe included in this northern 

 area so subject to change, since Cape Chelyuskin lies 600 miles 

 north of the Arctic Circle, while many Tertiary islands must 

 have existed that have since disappeared. It is all the while 

 to be borne in mind that broken land areas at the north are 

 likely to have been important factors in the faunal and floral 

 changes there taking place. 



It is to be added that it is not impossible, and even seem- 

 ingly probable, that the Antarctic area represented a minor 

 dispersion center facing the apices of the triangular continental 

 masses whence certain peculiar elements of the South Ameri- 

 can, African and Australian faunae may have originally sprung. 



More recently a masterful resume of the facts relating to 

 the impressive synchronous succession of similar vertebrate 

 faunae in Europe and America has been given by Dr. J. L. 

 Wortman,^ and to him belongs the merit of first having pointed 

 out its great extent and bearing on the question of the polar 

 origin of the main ancestral mammalian stocks. Although the 



* Studies of Eocene Mammalia in the Marsh Collection, Peabody Museum, 

 this Journal, vols, xi-xv, 1901, 1903. 



