E. V. Wulff —196— Historical Plant Geography 



which assumes the form of a long, narrow strip, following the 30th 

 parallel of Wegener's reconstruction across the entire breadth of the 

 Eurasian continent from western Europe to Japan. As it approaches 

 the Pacific Coast, the area lies somewhat to the north of this 30th 

 parallel. The change to an ever colder climate, that was initiated 

 toward the end of the Tertiary period, led to the dying out of the tulip 

 tree over a large part of its area. By the Pliocene stage we already 

 have two widely-separated areas, one in southeastern Asia and the 

 other in the southern part of western Europe. These two areas served 

 as centers of origin of the two present-day vicarious species. The 

 Asiatic area gave rise to L. chinense while the European area, accord- 

 ing to paleobotanic data, was presumably the place of origin of L. 

 Tulipifera. There is one circumstance that tends to cast doubt on this 

 explanation of the origin of these species, viz., that the latest fossil 

 remains of leaves of L. Procaccini in Europe are those of the beginning 

 of the Quaternary period, found in France. After this Liriodendron 

 entirely disappears from Europe. On the Atlantic Coast of North 

 America, on the other hand, after a prolonged lapse (during the whole 

 of the Tertiary period), there are again found Quaternary fossil re- 

 mains of the species L. Procaccini. 



In explanation of these puzzhng facts several suppositions may be 

 advanced. Of these the most plausible is that during the Tertiary 

 period in North America the area of this genus was shifted consider- 

 ably to the north, where it {persisted until the end of the Tertiary 

 period, when, due to the climate then becoming much colder, it mi- 

 grated back to the south to its former location. Although fossil re- 

 mains of Liriodendron have not yet been found in Tertiary deposits of 

 North America, it seems difficult to assume that this genus completely 

 disappeared during this period from the flora of America. It is to be 

 hoped that future paleobotanic findings will solve this enigma. 



According to data of I. G. Knoring of the Botanical Institute of 

 the Academy of Sciences of the U.S.S.R. (unpublished paper), there 

 exists similar close correspondence between changes in the area of the 

 genus Liquidambar and the changes in climatic zonation hypothesized 

 by KoppEN and Wegener. 



In addition to these data on the distribution of separate families 

 and genera, analyses of entire floras and studies of the history of their 

 development have provided numerous instances showing Wegener's 

 theory of continental drift to be in full accord with the data on the 

 migrations and interrelations of these floras in past geological periods. 

 The material of such studies is too voluminous to be presented here 

 even in summary form. It will be treated at considerable length in the 

 author's forthcoming book, "A History of Floras" (to be published by 

 the Academy of Sciences of the U.S.S.R.). 



Phjrtogeographical Arguments Against the Theory of Continental 

 Drift: — The theory of continental drift has been subjected to criticism 

 on the part of some paleobotanists and phytogeographers. But the 

 arguments advanced against it are comparatively few in number. 

 Among the most important are those expressed by Diels (1928, 1934, 

 1936), which may be summarized in the following three propositions: 



