INTRODUCTION 27 



squirrel may throw some light on the post-glacial chronology of 

 events on Long Island. This pine is found in the region surround- 

 ing the pine-barrens, but is unknown, or very rare in them. Pinus 

 rigida, the predominant tree of the barrens, is common on Long 

 Island, but Pinus echinata mentioned above and the red squirrel 

 are not known on the island.* From the geological outline given 

 above it is very probable that P. echinata must have occupied the 

 region surrounding the pine-barrens long after the last effects of 

 the ice were past. This may also have been true of the red 

 squirrel. At any rate, after a large post-glacial migration of 

 alleged "pine-barren" plants, the avenue of migration must have 

 been broken. The discontinuance of this passageway must, it 

 seems to me, in all probability have been the controlling factor 

 in the failure of Pinus echinata and the red squirrel to reach 

 Long Island. It is curious in this connection that the pine, but 

 not the animal, are found on Staten Island. There are, of course, 

 many more species than this pine, which apparently reach their 

 northern distribution point in the region surrounding Beacon Hill, 

 or in Staten Island, never having been reported from Long Island. 

 It seems probable that they came northward in post-glacial times, 

 too late to avail themselves of the already destroyed avenue of 

 migration. The following list gives some idea of the variety of 

 plants that are found on Staten Island but are not definitely known 

 on Long Island. That all these cases are attributable to the 

 agency sketched above, may be doubtful, but at any rate the list 

 is suggestive of what variation there is in the flora of the two islands. 



Species Found on Staten Island But Not Known from Long Island 



Filix fragilis, Bromus purgans, 



Dryopteris Goldieana, Carex striatula, 



Pinus virginiana, Arisaema Dracontium, 



Pinus echinata, Lemna trisulca, 



Panicum polyanthes, Wolffia columbiana, 



Panicum commutatum, Helonias bidlata, 



Agrostis Schweinitzii, Corylus rostrata?, 



Panicularia borealis, Asarum canadense, 



Panicularia septentrionalis, Coptis trifolia, 



* The reported occurrence, also, of Pinus virginiana in Suffolk Co., L. I., by Miller 

 and Young has not been verified. 



