The study of Geoloi^y indicates that at some time, 

 in the early history of the world, these continents were 

 more or less directly connected by continental bridges. 

 By means of these land connections, which made these 

 continents contiguous, it was possible for air-breathing 

 forms to migrate from one continent to another. Con- 

 sequent Geological changes isolated certain of these 

 continents, while on the other hand, land areas that 

 were formerly more or less isolated were connected 

 through the shifting of the oceans. Combined with 

 these changes in land and water areas were constant 

 variations of climatic conditions, such as were pro- 

 duced through the movement of glaciers and through 

 the elevation and lowering of land areas by upheaval. 

 The existing animal forms were, therefore, forced to 

 endure new and ever-changing conditions and, in the 

 process of evolution and adaptation, a multitude of 

 kinds resulted. 



As is to be expected, many animals were unable to 

 meet the situation and, failing in their Biotic response 

 to the new environment, they perished. Others which 

 could migrate, probably did so, while still others were 

 forced to modify their structures and habits of living. 



These conditions, ofttimes changing from a warm 

 climate, with its associated abundance of a particular 

 vegetation, to a comparatively frigid one, necessitated 

 either adaptation or migration. When certain forms 

 could do neither, they p-assed from the living picture 

 and became extinct. Others were able to move from 

 unfavorable situations and today we see their descend- 

 ants holding forth in a tropical setting which, although 

 somewhat modified, is similar in many respects to that 

 of their predecessors. Others met the new demands 



-^ 17 J^ 



