WALKER ON ORIGIN AND DISTRIBUTION OF MOLLUSCA. 53 



during the Mesozoic and Tertiai y periods this great sea was by the 

 general continental elevation separated from the great open ocean and be- 

 came first brackish and then fresh water, and finally after the elevation 

 of the Kocky mountains in its midst, was wholly drained off or evaporated, 

 leaving the great western plains of the present daj as surface evidence 

 of its former existence. The existence of this great body of water, 

 stretching from what is now the CJulf of Mexico to the Arctic ocean, salt 

 when first separated from the primeval ocean by the continental eleva- 

 tion at the north and south, and gradually becoming a series of great 

 fresh water lakes, is perhaps the most important factor in evolutionary 

 history of our mollusca. For not only in its waters were developed the 

 ancestral types of nearly all of our existing fresh water forms, an almost 

 unbroken series of which, from the earliest Mesozoic times to the present 

 have been preserved in its sedimentary deposits, but as we shall see, it 

 has also played a most important part in limiting the immigration from 

 other regions. 



And in connection with this, it must be remembered that the great 

 coast ranges of the Sierra Nevada on the west and the Appalachians on 

 the east have been in existence substantially as they now are from the 

 earliest times, and in this way, must have to no small degree affected 

 not only the distribution of the great faunas of the prehistoric ages, but 

 that of many of our recent species. 



The third great factor, which in past ages has influenced the distribu- 

 tion of our molluscan fauna, was the glacial epoch toward the close of the 

 Tertiary period. The advance of the post jdeiocene ice sheet, not only 

 wiped out of existence all forms of life, which were unable to escape be 

 fore it, but the influence of its attendant low temperature extending far 

 beyond the line of the ice itself, absolutely extinguished the great fauna 

 of southern forms which had poured into North America from South Am- 

 erica in early Tertiary times, and were unable to withstand the radical 

 change in the climate, the bones of whose gigantic mammals now alone 

 remain to astonish the beholder and to play their part in the elucidation 

 of the world's history. That the gradual advance of the ice must neces- 

 sarily have sounded the death knell for all animal life through the 

 entire northern portion of the continent, as far south as the valley of the 

 Ohio, can be easily appreciated when it is remembered that its height 

 is estimated to have been, at least, nine thousand feet, and that when 

 it receded it left the lowlands of New England and the northern states 

 buried under a bed of boulder clay and glacial drift from ten to two 

 hundred feet deep. It is estimated by Prof. Newberry that the average 

 depth of the drift in the state of Ohio is, at least, sixty feet. 



T'pon the recedence of the glacier, the fall of the waters left the sur- 

 face of the continent as we now find it, and the scattered remnants of 

 the Tertiary fauna along its southern border were enabled to spread out 

 over the new land and to establish the fauna of the continent as it exists 

 today. 



More than twenty-five hundred years ago the philosophers of ancient 

 Greece, influenced no doubt by the teeming life, which swarmed in their 

 native seas, taught their disciples that all life came from the ocean, and 

 today the exponent of modern scientific thought can but reaffirm the 

 happy speculation of these wise men of old and assert it as one of the 



