Chapter X —197— Historical Causes 



1. The flora of the Atlantic Coast of North America is linked not 

 with the flora of Europe but with that of eastern Asia; 



2. The relic flora of Europe is related to the Asiatic flora and does 

 not bear any American features; 



J. The autochthonous flora of Australia bears a paleotropic char- 

 acter. 



On the basis of these three propositions Diels considers that 

 Wegener's hypothesis is incompatible with the data of botanical ge- 

 ography. Diels' arguments were replied to at length in a paper by 

 Irmscher (1929), to which we refer the reader for a detailed criticism. 

 We wish here only to state that Diels' categorical conclusion is, in our 

 opinion, not convincing, since the relatively meager data advanced by 

 him are refuted by counter-arguments, the number of which is con- 

 stantly increasing. Thus, recent analyses of the elements of the flora 

 of Australasia (Schwarz, 1928; Lam, 1934) have elucidated the re- 

 lationships between these elements and have constituted a basis for 

 conclusions quite at variance with those of Diels. Moreover, the 

 latter's arguments do not by any means cover all the phytogeographical 

 problems that are solved by the theory of continental drift. In answer 

 to his criticism it is possible to present a considerably greater number 

 of biogeographical facts that are not contradictory to but confirmatory of 

 Wegener's theory. Lastly, Diels' arguments may be answered in his 

 own words: ,,Eine Theorie der Erdgestaltung soil uns nicht ein beliebiges 

 Areal erklaren, sie muss die grossen Ziige der Florenbildung verstandlich 

 machen" (1928, p. 55). 



Considerably more serious are the arguments against Wegener's 

 theory based on problems regarding the Pacific Ocean, in which field, 

 from the point of view of biogeography, this theory requires certain 

 modifications. According to Wegener, the Pacific Ocean has existed 

 during the entire history of the earth, and its islands constitute land 

 fragments left at the time of the formation of this ocean in pre-geologic 

 times. Assuming such an origin of these islands, it follows that all 

 their fauna and flora are adventive in character. However, there are 

 numerous biogeographical facts that refute such a conclusion. Conse- 

 quently, many investigators of the organic life, particularly plant life, 

 of the Pacific islands — e.g., Beccari (1886), Holdhaus (1909), Halller 

 (1912), Skottsberg (1925, 1928), von Ihering (1927), Guillaumin 

 (1928), and Campbell (1928) — consider that there must have existed 

 trans-Pacific connections between America and Asia. These views 

 have also found support on the part of a number of geologists (Haug, 

 1912; Gregory, 1930). 



A similar view is advanced by Du Rietz (193 i) in his interesting 

 paper on two new species of Euphrasia from the Philippines, E. 

 philippinensis and E. Merrillii. The genus Euphrasia has two widely- 

 separated centers of concentration of species, one in the northern 

 hemisphere, in Europe, Asia, and the northern part of North America, 

 and the other in the southern hemisphere, in Australia and the archi- 

 pelago of islands to the north. New Zealand, and the southern part of 

 South America. Between the groups of species located in North and 

 South America, over the entire length of the two American continents, 

 there are no connecting links. The South American species, however, 



