528 CRITICAL PERIODS IN THE HISTORY OF THE EARTH. 



For example : in America during the Glacial epoch, coincidently with 

 the rigorous climate, there was an elevation of the continent, greatest in 

 region of high latitude, but also probably great along the line of the Mis- 

 sissippi Eiver; for in this region it extended southward even to and beyond 

 the shores of the Gulf. Prof. Hilgard has shown that the elevation at the 

 mouth of the Mississippi Eiver was at least four hundred and fifty to five 

 hundred feet above the present condition. Until the Glacial times the two 

 Americas were certainly separated by sea in the region of the Isthmus, as 

 shown by the Tertiary deposits there. This barrier was removed by up- 

 heaval during the Glacial epoch, and a far broader connection existed then 

 than now. Through this open gate-way came the fauna of South America, 

 especially the great Edentates, into Korth America. Similarly a broad con- 

 nection then existed between America and Asia in the regions of the shal- 

 low sea between the Aleutian Isles and Behring Strait. Through this gate- 

 way came an invasion from Asia,^includiag probably the mammoth. With 

 this invasion probably came also man. It seems probable, therefore, that 

 the earliest remains of man in America will be found on the Pacific coast. 



Also the great Pliocene lake, which stretched from near the shores of 

 the Gulf far into British America, and possibly into arctic regions, and 

 formed a more or less complete barrier to the mammalian fauna east and 

 west was abolished by upheaval, and free communication was established. 

 It is impossible that all these changes of climate and all these migrations 

 partly enforced by changes of climate and partly permitted by re- 

 moval of barriers, and in this latter case especially attended with the fierc- 

 est struggle for life, should not pi'oduce i-apid and profound changes in the 

 mammalian fauna. 



In Europe the process has been more accurately studied and is better 

 known. In Quaternary times at least four different mammalian faunae 

 struggled for mastery of European soil. (1.) The Pliocene autochthones. 

 (2.) Invasions from Africa by opening of gate-ways through the Mediter- 

 ranean: one by way of Italy, Sicily, and Malta and one by Gibraltar, both 

 of which have been again closed. (3.) Invasions from Asia, by removal of 

 a great sea barrier connecting the Black and Caspian seas with the Arctic 

 Ocean. This gate-way has remained open ever since. (4.) Invasions from 

 arctic regions, enforced by changes of climate. Probably more than one 

 such invasion took place; certainly, one occurred during the second Glacial 

 epoch. The final »esult of all these climatic changes and these struggles 

 for mastery was that the Pliocene autochthones, adapted to a more genial 

 climate, were mostly destroyed or else driven southward with some change 

 into Africa : the xVfrican invaders were driven back also into Africa, and 

 with some Pliocene autocthones isolated there by subsidence in the Mediter- 

 ranean region closing the southern gate-ways, and still exist there under 

 slightl}^ modified forms ; the Arctic invaders were again driven northward 

 by return of more genial climate, and there exist to this daj^ ; while the 

 Asiatics remain masters of the field, though greatly modified by the conflict. 



