Minnesota Plant Life. 17 



make their escape to the temperate climate beyond the influ- 

 ence of the ice, and under these conditions most of them must 

 have perished as wretchedly as did so many of the troops of 

 Hannibal when they crossed the Alps. Moreover, when the gla- 

 cial period came to an end in Europe and Asia it was difficult 

 for plants to return over the mountain-passes, and as a result, 

 these continents are tenanted by a less diversified vegetation 

 than that of North America. 



The longitudinal mountain ranges of the New World have 

 rather aided the movements of plants than hindered them, for 

 they have assisted northern plants to find their way along high 

 altitudes to constantly lower latitudes, while at the various stages 

 of their journey such plants have enjoyed the opportunity of 

 climbing down the mountain sides and out upon the plains, if 

 they were able to accommodate themselves to the higher tem- 

 perature. This movement has taken place not only on the 

 western side of the continent, but also along the Alleghenies. 

 Yet owing to the greater height of the western range it is found 

 that northern genera of plants like some of the roses and willows 

 have developed more abundantly toward the southwest than 

 toward the southeast, simply because they have followed an 

 easier path along the cool high ridges of the Rockies than along 

 the warmer, lower Appalachian range. By these two mountain 

 ranges, lying one to the east and the other to the west of Minne- 

 sota, some slight influence has no doubt been exerted upon 

 plant migration, both in the prairie and in the forest region of 

 the state. But this effect must have been stronger in the prairie 

 region ; for the forest plants from the east and north could enter 

 as easily from the north and would not need to depend upon 

 any lateral movement. From the west, however, where the 

 plains rise gently to the mountains, many plants which had 

 found an asylum on the sides of peaks and escarpments must 

 have, in the last ten thousand years, slowly crept down into the 

 plains and there developed habits and structures which persist 

 to this day. 



3 



