Chapter X —199— Historical C auses 



As an authoritative reply to Du Rietz we may cite the words of 

 Seward (1934), who himself is by no means inclined to accept Weg- 

 ener's theory unconditionally. As a result of a study of fossil woody 

 plants of the Kerguelen Archipelago, he came to the conclusion that : — 



"We are not exclusively concerned with possibiHties of plant-dispersal and the stocking 

 of areas that are now oceanic islands; there is also a cUmatic problem. Phytogeography 

 IS not only intimately connected with geology; it may receive substantial assistance from 

 geophysics. Improved methods of soundings have shown that some parts of the ocean- 

 floor are counterparts of mountainous regions of continents rather than submarine plains- 

 former isthmian Knks from continent to continent . . . may be imagined with reasonable' 

 justification, but it is very doubtful, if they in themselves wiU supply adequate explana- 

 tions of climatic conditions necessitated by extinct floras. Vertical movements alone are 

 insufficient; lateral displacement of continents seems to be an almost necessary assumption. 

 It IS stated that recent soundings reveal contours on the sea-floor comparable with those of 

 contments; and that the South AntiUean arc, geologicaUy and geographically, shows a 

 mixture of oceanic and continental characters. These and other arguments have been ad- 

 vanced against the acceptance of Wegener's theory with its ' absurd phytogeographical 

 consequences', which Do Rietz (1931) considers are demonstrated by Diels and other 

 authors whom he quotes. Though it may be conceded that the difficulties presented by 

 the past and present distribution of plants cannot be satisfactorily disposed of by the ac- 

 ceptance of WEGE>fER's theory as he propounded it, this concession does not preclude 

 behef in some form of continental drift. 



"The Wegener hypothesis, despite the serious criticism which it has raised, appeals 

 strongly to the imagmation. ... It is by no means improbable that solutions of some of 

 the many problems of Plant Geography— both past and present— wiU be found, not in 

 the raising of foundered continents, but through the acceptance of the mobility of the 

 earth's crust, as a factor not merely imagined but substantiated by evidence which, it may 

 be suggested, will eventually be provided" (p. 736). 



All these considerations show that by far not all the puzzling prob- 

 lems in the distribution of plants on the islands of the Pacific Ocean 

 and the countries bordering on this ocean may be considered solved. 

 Undoubtedly there also existed continental connections in the southern 

 part of the Pacific similar to the connection between America and Asia 

 in the region of the Bering Straits. Nevertheless, it is impossible to 

 assume the former existence either of a lost continent occupying the 

 basin of the Pacific Ocean or of a direct contact between America and 

 Asia, as in the case of America and Euro-Africa. The history of the 

 Pacific Ocean is entirely different from that of the Atlantic. 



Although we have not enumerated here all the points in Wegener's 

 theory that are not in accord with biogeographical data, we shall not 

 pursue this further. In our opinion they do not suffice to discard 

 Wegener's theory as incorrect but only indicate that in this theory 

 there is still much that requires further elaboration and modification. 

 In any event its significance for biogeography is beyond doubt. It has 

 thrown light on a number of hitherto entirely incomprehensible mo- 

 ments in the geography of plants. It has shown us new ways out from 

 that bhnd alley into which we were led by the mutually exclusive 

 theories of land-bridges and of the permanence of oceans and con- 

 tinents. Even though, as Kubart (1926) pointed out, continental drift 

 is not the sole factor determining the present areas of relic species, and 

 even though Wegener's theory still requires additions and corrections, 

 it, nevertheless, constitutes the only plausible working hypothesis upon 

 which the historical plant geographer may base his conclusions. 



