468 GENERAL MORPHOLOGY OF PLANTS 



in their southward movement; probably a large percentage of 

 them survived. In North America the mountain chains were 

 parallel with the migratory movement and permitted the south- 

 ward 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 movement in populating the conti- 

 nent after the disappearance of the ice-sheet. Likewise the 

 Alps and Pyrenees presented a barrier to the northern movement 

 of the plants of southern Europe. The Rocky Mountains afford 

 a barrier between the flora of the Pacific Coast and the country to 

 the east. 



637. Conflict of species in migration. This is one of the 

 most noticeable features in plant migration. With the means for 

 movement with which plants are provided, together with the 

 pressures exerted, forcing them to move, they are constantly 

 reaching out for new territory and struggling to hold that which 

 they already occupy. The competition becomes severe because 

 of the large number of species which are adapted to live under 

 similar conditions. Some have compared the struggles of plants 

 to occupy new territory, or to maintain their hold upon their own, 

 to the competition which exists among human societies. Every 

 plant must be able to propagate itself and to hold territory in 

 competition not only with climatic conditions, but also with 

 other plants entering the same region. It must either hold its 

 territory or cede it to its more successful rivals. Thus plants 

 which are adapted to live under the conditions of a given terri- 

 tory are those which survive, while the weaker ones are driven 

 out, or exterminated, or occupy a very subordinate place in the 

 society. 



