52 MICHIGAN ACADEMY OP SCIENCE. 



a few words in regard to the geological history of the continent and its 

 relation to the subject may not be out of place. 



By the general concurrence of scientific opinion, the sometime theories 

 of the existence of an Atlantis or other ancient land connection across 

 either the Atlantic or Pacific oceans between the old and new worlds have 

 been put aside as wholly untenable, and "the general permanence of 

 what are now the great continents and deep oceans" is now generally 

 accepted as an established fact. The great changes which from time to 

 time in the world's history have occurred from the constantly recurring 

 submergence of the land beneath the sea and it's subsequent unheavals, 

 are believed to have only changed the configuration of the surface of 

 these ancient continents, and from time to time altered their area and 

 extent. There is no reason to believe that, from the time when the dry 

 land first appeared above the surface of the palaeozoic sea, there has ever 

 been a period when any of the great continental areas have been wholly 

 submerged. There has always been a refuge where at least a remnant 

 of the existing fauna has been preserved, that might again under favor- 

 able auspices, though with changed surroundings, re-people the earth. 

 But. while modern geology fails to bridge the Atlantic and Pacific, it is 

 free to admit what palaeontology claims must have been the fact, that at 

 certain periods there has been a land connection between the old and the 

 new world. There can be no doubt but that in ages past there has been 

 from time to time such an elevation in the extreme north as to unite 

 Asia and what is now Alaska. 



Whether there has ever been a similar Antarctic continent uniting 

 Africa and South America with perhaps New Zealand is not yet generally 

 admitted. With one exception perhaps, it is a question which has no 

 bearing upon the scope of the present discussion, and it may be passed 

 by with the remark that such an extension of the earth surface is con- 

 tended for many able authorities, and that it is a hypothesis which would 

 solve some of the most perplexing questions now before the zoogeograph- 

 ers. 



As all of the palaeozoic strata are considered to be of marine origin, 

 with the possible exception of the coal deposits of the Carboniferous 

 age, in which are found the earliest known non-marine mollusca, the 

 following account by C. A. White of the United States Geological Sur- 

 vey will be a sufficient statement of the condition of the continent at 

 the time when the non-marine fauna first appeared. "East of west longi- 

 tude 95° (the western part of the Mississippi valley), North America is 

 mainly occupied by Paleozoic and Archaean rocks, as is also a large 

 area which extends northward and southward through western North 

 America, the eastern border of which is not far from the 113th meridian 

 of west longitude. These two great areas are taken to represent approxi- 

 mately the outline and extent of the principal portions of the North Amer- 

 ican continent that were above the level of the sea at the beginning of 

 the Mesozoic time. A broad expanse of Mesozoic sea then stretched 

 between these two continental factors, which were finally united by a gen- 

 eral continental elevation and the consequent recedence of the sea. This 

 elevation was not — properlj- speaking — catastrophal, but gradual and 

 oscillatory." Without going into detail in regard to gradual elevation 

 of the continental area, it is sufficient for our present purpose to add, that 



