Chapter I 



HISTORICAL PLANT GEOGRAPHY — SCOPE, 



RELATION TO ALLIED SCIENCES, METHODS 



OF INVESTIGATION 



Scope and Name of the Science: — Historical plant geography has as 

 its aim the study of the distribution of species of plants now existing 

 and, on the basis of their present and past areas, the elucidation of the 

 origin and history of development of floras, which, in turn, gives us a 

 key to an understanding of the earth's history. In this respect histori- 

 cal geography of plants and animals is a direct continuation of his- 

 torical geology. The latter science bases its conclusions on a study of 

 fossil organisms, both of animal and plant origin. Consequently, its 

 penetration into a knowledge of the history of our planet carries us no 

 further than the Tertiary or the beginning of the Quaternary Period. 

 From then on the further study of the past fates of the earth passes 

 over to the biologist, who, on the basis of the present distribution of 

 living organisms and of data regarding their past habitats, determines 

 those changes which occurred in that complex combination of diverse 

 factors as a result of the interaction by which the areas of these organ- 

 isms have been established. By an analysis of these changes he con- 

 tributes to the task of reconstructing the past aspect of the earth and 

 its history. 



The vegetation of our planet is under the constant influence of the 

 most diverse factors facilitating or hampering its development. These 

 factors affecting the earth's vegetation not only functioned in past 

 geological periods, when changes in the configuration and location of 

 the continents, the formation of new mountain systems, the trans- 

 gression and regression of seas, and changes in climatic conditions 

 called forth changes in the distribution of plants leading to the creation 

 of their present areas; these factors, to which has been added another 

 major one, man's activities, have also continued to function in the 

 present period of the earth's history. 



The more recent changes in the earth's vegetation — the disappear- 

 ance of forests, the formation of deserts, the draining of swamps and 

 changes in their vegetation, the crowding out of some species by others, 

 the destruction or dying out of single species or whole floras and their 

 replacement by cultivated vegetation — which may frequently be traced 

 from historical data likewise constitute one of the chapters in historical 

 plant geography. 



This conception of the scope of this branch of botanical geography 

 leads us also to the name by which we designate it, "historical plant 

 geography". The introduction of this term should, we believe, be 

 ascribed in part to Stromeyer but chiefly to Schouw, who used it in 

 his "Grundziige einer allgemeinen Pflanzengeographie " (1822) to desig- 



