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Sonic such action as the former of these two examples seems 

 clearly to be under way, for, as has been shown, the New Jersey 

 Pine Barrens are at present surrounded by a more advanced 

 llora which is pushing in from all sides wherever conditions are 

 favorable, and man is rendering no small assistance in the move- 

 ment. Both east and west of the Pine Barrens there can be 

 readily detected a northern and southern element apparently ad- 

 vancing in opposite directions in a common effort to conquer the 

 Pine Barrens. The more or less complex character of the Pine 

 Barren flora to-day as regards its origin is apparently due to a 

 combination of movements such as described above. 



Of course, great physical changes in the earth's surface in 

 geologic time must have had tremendous effect upon the flora, 

 usually producing climatic changes which acted directly upon 

 plant life. Such changes, of course, were responsible for the 

 great fall in temperature coincident with the glacial epoch. Sub- 

 sidences, too, which are known to have occurred at different 

 periods, must have entirely exterminated the flora of large areas. 



Just how far we can correlate existing conditions of plant dis- 

 tribution with geologic changes it is difficult to say. Most at- 

 tempts of this sort seem to suppose a definiteness of knowledge 

 of the time relationship of various geologic phenomena which we 

 do not possess, and there is a tendency to assume constancy in 

 the character of the flora of certain areas, while that of contig- 

 uous areas is undergoing tremendous changes. Such hypotheses, 

 so far as they attempt detailed explanations, are purely con- 

 jectural. 



Some facts, however, are clear. We know that the coastal 

 plain was submerged at a time when the elevated Piedmont 

 region to the west must have been covered with vegetation, and 

 that plant life on the region north of the terminal moraine must 

 have been for the most part exterminated during the glacial 

 epoch. Therefore, the area between the coastal plain and the 

 terminal moraine must have been continuously covered with 

 plants for a much longer period than have these two regions 

 themselves. When the coastal plain was elevated above the sea 

 it must have received its flora from the contiguous country to 

 the west or southwest. Furthermore, the several partial sub- 



