PREFACE. 



The differences in plant life which exist between distant or even 

 between nearby locahties must have come under the notice of man 

 in the earhest semiciviHzed stages of his existence. Human depend- 

 ence upon the products of the vegetable kingdom has served to 

 maintain throughout all historic time a vivid realization of the 

 vegetational differences encountered with changes of latitude, alti- 

 tude, and proximity to the sea. The later stages of modern civiliza- 

 tion have done extremely little to liberate man from his dependence 

 upon plants, although the development of methods for the preserva- 

 tion and transportation of food has given him greater freedom of 

 movement into the jungle, the polar regions, the desert, and the 

 modern city. 



Many of the activities of the last 150 years have been such as to 

 increase our interest in the distribution of plants and in the nature of 

 the plant populations which characterize different regions, different 

 soils, or different topographic situations. Innumerable bands of ex- 

 plorers and collectors have penetrated all parts of the world, bringing 

 back materials upon which we have been able to base a knowledge of 

 the flora and the larger aspects of the vegetation of all but the most 

 inaccessible portions of the globe. The extensive introduction of 

 economic and ornamental plants into new regions and even into new 

 continents has awakened an interest in the possibility of still further 

 introductions and in a study of the causes of the success or failure of 

 such as have been made. The increasing value of all the products of 

 the forest has led to the planting of trees on a large scale within their 

 native regions and to the experimental introduction of trees from dis- 

 tant countries, as well as to attempts to improve natural forest stands. 

 The increasing population of the world has augmented the value of its 

 agricultural lands and has given importance to the study of the proper- 

 ties of the soil in their relation to plants. The search for new agri- 

 cultural regions and for crop plants adapted to the conditions in newly 

 settled areas has led to an interest in natural plant growth as an index 

 of the most promising soils or of the most suitable crops to be cultivated. 



Hand in hand with this widening utihzation of the plant products 

 of the world has gone a rapid development of the scientific study of 

 plants in relation to their natural environment. During the first half 

 of the nineteenth century there was a rapid accumulation of facts 

 regarding the composition of the floras of the outlying portions of the 

 earth, and these facts were almost as rapidly marshaled into an ordered 

 knowledge of the great fioristic regions. In this immense task the 

 names of Humboldt, Schouw, Grisebach, de Candolle, Hooker, and 

 Engler are intimately associated with the greatest accomplishments. 

 The interests of plant geography in this stage of its development were 



IX 



