E. V. Wulff — 4 — Historical Plant Geography 



Relation to Paleobotany: — Studying the present distribution of 

 plants, historical plant geography cannot but investigate their past 

 distribution, since only on the basis of the latter can their present areas 

 be understood. Unfortunately, fossil plants are to be found in very 

 small numbers and often in such a condition that it is impossible to 

 identify them. Nevertheless, even the little that we know about the 

 vegetation of earlier geological periods, particularly of the Tertiary 

 Period, gives us very valuable indications as to the former distribution 

 of genera and species of plants, some of which are still in existence, and 

 this in turn gives a basis for determining the cUmates and distribution 

 of land areas in those remote times. Consequently, paleobotany is a 

 necessary basis for historical plant geography, but the latter does not 

 cover the same ground as the former, despite the closeness of the aims 

 of research in the two sciences. Paleobotany makes a study not only 

 of the taxonomy, morphology, biology, and geography of fossil plants 

 but also of the history of development of the floras of former geological 

 periods, in this latter respect constituting a science parallel to that of 

 the historical geography of plants now living. Historical plant geog- 

 raphy begins its work at the point where paleobotany leaves o_ff. 



Relation to Phylogenetic Taxonomy: — Phylogenetic taxonomy of 

 plants has as its aim the arrangement of plants (now or formerly exist- 

 ing) in a system based on the degree of their kinship and the history 

 of their development. In connection with this aim modern plant 

 taxonomy should use fully objective methods of determining relation- 

 ships. At present no one longer doubts that the morphological method 

 of comparison alone cannot give sufficiently trustworthy data as to the 

 relationship of taxonomic units. Consequently, there have been 

 advanced several objective methods of establishing the relationship 

 between species and forms being studied. Among such methods is that 

 introduced by Wettstein (1898) under the name "geographical- 

 morphological method of plant taxonomy". This method has proved 

 to be exceptionally fruitful, and to the present time is widely used in 

 monographs on plant taxonomy. It is founded on the supposition, 

 based on a number of extensive investigations, that there exist close 

 interrelations between species-formation and' habitat conditions. 

 Habitat conditions vary not only in relation to time but also in re- 

 lation to space. Hence, it is quite clear that species arising as a result 

 of adaptation to or the effect of conditions characteristic of a definite 

 area should occupy this area. Consequently, on the basis of the dis- 

 tribution of plants one may draw conclusions as to their origin. 



Species may be divided into three main groups according to the 

 conditions and time of their origin. To the first group belong those 

 species which have arisen comparatively recently as a result of adap- 

 tation to new habitat conditions, to which they have been subjected 

 either due to their migration beyond the limits of their initial area or 

 due to changes in conditions within part of their area. Such species 

 are, presumably, very closely related to the species from which they 

 arose, inhabit contiguous but not overlapping areas, and are linked 

 by a number of transitional forms not of hybrid origin. 



To the second group belong species of more ancient origin. Such 



