PLANT MIGRATION: 513 



and here a lateral pressure is exerted which crowds the plants 

 to the west and east. Pressures also exist in the borders of arid 

 regions. The fertility of aggressive species wherever they occur 

 tends to produce pressures in all directions. 



983. Barriers to plant migration. There are a number of 

 barriers which plants meet in their migration over the surface of 

 the earth. In general terms we might speak of four when look- 

 ing at the world as a whole, ist. Kinds of climate: regions of 

 great heat or cold, of dryness or moisture, etc. All these regions 

 oppose obstacles to the entrance and passage of plants which 

 are accustomed to live in different climates. 2d. Kinds of soil; 

 for example, the alkaline deserts and the great salt steppes pre- 

 sent effectual barriers to the passage of plants not provided with 

 adaptations which would enable them to live under such ex- 

 treme conditions 3d. Discontinuity 0} land. Here bodies of 

 water present a barrier to the passage of plants from one conti- 

 nent to another, or from one island to another, which are 

 separated by broad lakes or seas. A good illustration of this is 

 shown in the relation of the continents of the southern hemi- 

 sphere as compared with those of the northern hemisphere 

 already pointed out. 4th. Mountain chains. High mountain 

 chains, because of the great cold, often form impassable barriers 

 for plants; good illustrations of this are shown in a comparison 

 of the number of species of plants in Europe and North America 

 and their distribution. Under the high climatic pressures which 

 existed, for example, in glacial times, the plants of North Amer- 

 ica met with no barrier in their southward movement; prob- 

 ably a large percentage of them survived. In North America 

 the mountain chains were parallel with the migratory movement 

 and permitted the southward flow and return of the species. 

 On the contrary on the continent of Europe during the same 

 period in their southward movement the plants met with an 

 impassable barrier in the Alps and Pyrenees Mountains, which 

 extend east and west across southern Europe. Many of the 

 species thus perished and were not left to join in the return move- 

 ment in populating the continent after the disappearance of 



