E. V. Wulff —8— Historical Plant Geography 



four genera of this family. The trend of geographical distribution of 

 these species — from the Malay Archipelago toward the west in the 

 direction of India and toward the east in the direction of New Guinea 

 • — indicates that the Malay Archipelago should be considered the 

 initial center of development of the Sapotaceae. 



The botanico-geographical method is based on a study of the areas 

 of the species of a given flora, not only within the limits of the latter 

 but, which is most important, in all their entirety. As a result of such 

 an investigation, the centers of concentration of species may be es- 

 tablished, which point to the centers (primary or secondary) of de- 

 velopment of genera. Moreover, by establishing the character of an 

 area, it is possible to determine of what ecological, geographical, and 

 historical elements a given flora is composed. Combining these data 

 with those obtained from a taxonomic study makes it possible to sub- 

 divide the estabhshed geographical elements — according to the cen- 

 ters of origin of the areas of the various species — into a number of 

 groups reflecting the genesis and process of development of the flora 

 under investigation, the degree of its autochthonism, i.e., the extent to 

 which the species composing it have originated and developed within 

 the territory occupied by the given flora, the extent to which such 

 species have migrated from neighboring floras, and the time and direc- 

 tion of these immigrations. A comparison of the flora under inves- 

 tigation, from the indicated points of view, with analogous floras of other 

 countries, makes it possible to elucidate the mutual relations of these, 

 floras, while the combined study of the floras of various countries and 

 regions gives a picture of the history of development of the flora of the 

 entire globe. 



Lastly, changes in vegetation during the most recent period of the 

 earth's history, when it has been affected by man's activities, are 

 studied both on the basis of data regarding the interrelations of species 

 in plant associations and on the basis of historical documents and 

 material. A study of the ecology of species in different habitats within 

 their areas may elucidate the biological peculiarities of the initial types 

 and thereby indicate the direction of dispersal of a given genus or 

 species. 



In the case of all the above-mentioned indirect methods it is of ut- 

 most importance to study not separate species but a geographical series 

 of species that replace one another throughout the area of the genus to 

 which they belong. The difficulties confronting us in this field of 

 science, only the initial steps in whose development have so far been 

 made, are very great, but there are no grounds for regarding them as 

 insuperable. 



References: 



DiELS, L., 1921: Die Methoden der Phytographie und der Systematik der Pflanzen 

 (Abderhalden's Handb. d. biol. Arbeitsmethoden, Vol. 11, Part I, No. 2). 



EcKARDT, W., 1921: Die Palaoklimatologie, ihre Methoden und ihre Anwendung auf 

 die Palaobiologie (Abderhalden's Handb. d. biol. Arbeitsmethoden, Vol. 10, No. 3). 



Gams, H., 1926: Pflanzengeographie, Palaogeographie und Genetik (Petermann's 

 Geog. Mitteilungen). 



Karsten, G., 1922: Methoden der Pflanzengeographie (Abderhalden's Handb. d. 

 biol. Arbeitsmethoden, Vol. 11, Part I, No. 3). 



KoMAEOv, V. L., 1908: Introduction to the floras of China and Mongolia (In Russian; 

 Acta Horti PetropoUtani, Vol. 29). 



