E. V. Wulfif —6— Historical Plant Geography 



Relation to Paleoclimatology: — Climatic conditions constitute one 

 of tiie important component factors which, taken together, determine 

 the present distribution of plants. Of exceptional importance for 

 elucidating the history of the former distribution of plants and, conse- 

 quently, for an understanding of their present areas, is an acquaintance 

 with the climate of past geological periods, when the areas now under 

 investigation were formed. 



Conversely, paleoclimatology to a considerable degree bases its work 

 on biogeographical data, not only as regards separate species but also 

 as regards the distribution and character of entire floras. For paleo- 

 climatological research paleogeographical reconstructions are, as 

 EcKARDT (1921) has expressed it, "der wichtigste Lebensquell", and, 

 inasmuch as paleogeography is based on biogeographical data, paleo- 

 climatology is also linked with biogeography. 



Relation to Historical Geology: — From all the foregoing it is per- 

 fectly clear that, if the chief object of historical plant geography is to 

 explain the present distribution of plants on the basis of the history of 

 their past habitats, the geological history of the earth's surface, the 

 history of seas and lands, i.e., historical geology, must constitute an 

 important base for starting points in investigations into historical plant 

 geography. Data for biogeographical conclusions as to the former 

 character of now discontinuous areas of distribution of organisms, data 

 for hypotheses regarding former connections between the continents, 

 without which many factors in this distribution cannot be understood, 

 data as to the presence of seas where there is now land and land where 

 there are now seas, data as to the movement of glaciers and seas — in a 

 word, historical plant geography finds in historical geology all that 

 which reconstructs the history of the earth. Biogeographical conclu- 

 sions only find definite confirmation, when it is possible to base them 

 on a geological foundation, which historical geology alone can provide. 

 At the same time, biogeography, inasmuch as it is a source of paleo- 

 geographical structures, also contributes its bit to the work of his- 

 torical geology in deciphering the pages of the ancient manuscript of the 

 earth's history. 



Methods Employed in a Historico-Geographical Study of Floras: — 



The history of the development of any given flora may be established 

 on the basis of an accumulation of data, which may be amassed by all 

 the various modes of investigation which the present state of our 

 knowledge places at our disposal. The initial step should be to ac- 

 quaint ourselves with the geological, paleogeographical, and clima- 

 tological history of the territory of the flora or floras under study. 

 An analysis of present-day floras, based on a study of the areas of their 

 component species, should next be made both by the direct method of a 

 study of paleobotanical data and by a number of indirect methods, 

 such as the phylogenetic, botanico-geographical, ecologico-phyto- 

 coenological, and biotic. By the last-mentioned we mean a comparison 

 of the distribution of plants with that of animals or of the distribution 

 of plant hosts with that of their parasites. 



Historico-geographical conclusions should be based, first of all, on 



