84 OKIGIN OF LIFE IN AMEEICA 



The problem at issue is obviously not to be solved by the 

 application of purely geological reasoning. Dr. Brooks, in 

 his splendid contribution to the geology of Alaska, carefully 

 avoids any discussion of a former land connection with Asia. 

 In his brief statement that a land bridge between Asia and 

 North America could only have been utilised for Glacial or 

 post-Glacial migrations of Asiatic types, Professor Suess * 

 does not make us acquainted with the reasons for his supposi- 

 tion that the land connection did not also exist in Pliocene 

 times. He alludes to the striking circumstance that the 

 hypothetical land bridge was used apparently by Asiatic types 

 only, and leaves us wondering why American types should 

 not have taken a similar advantage of pouring into Asia. 

 That, however, is part of the problem which we must reserve 

 for a later stage of the discussion. 



Dr. Matthew f does not share Professor Suess' opinion, 

 for he indicates a wide land connection, and Professor Osborn 

 follows him in that respect, between North America .'and 

 north-eastern Asia in the Bering Sea region in Oligocene, 

 Miocene, Pliocene and Pleistocene times. 



As a rule the opinions of geologists on this problem are 

 based on biological evidence alone. It is the latter, there- 

 fore, that we have to rely upon. Curiously enough, Mr. 

 Knopf J maintains that all the stratigraphical testimony from 

 which conclusions of some positiveness can be drawn, record 

 only epochs of more widely-spread submergence and in- 

 creased separation of the continents, although he recognises 

 the strength of the palaeontological demands in favour of 

 more or less continuous inter -continental communications. 

 I shall endeavour, now, to critically examine the problem 

 from that point of view. I have already had several oppor- 

 tunities of expressing my own views on this subject (pp. 32 

 and 68), and these are entirely in favour of a geologically 

 recent Bering Strait land bridge (see Fig. 7). - But we require 

 to know more than this. We want evidence which will lead to 

 the determination of the geological age of the bridge and the 

 approximate date of its beginning and end. 



* Suess, E., " Antlitz der Erde," Vol. Til., p. 764. 



t Matthew, W. D., " Hypothetical Outlines of Continents." 



J Knopf, A., " Probable Tertiary Land Connection," p. 419. 



