514 RELATION TO ENVIRONMENT. 



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. 



983a. 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 be- 

 cause 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 soci- 

 eties. 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 best adapted to live under the conditions of a 

 given territory are those which survive, while the weaker ones 

 are driven out, or exterminated, or occupy a very subordinate 

 place in the society. 



