328 PLANTS, SEEDS, AND CURRENTS 



Application of the same Principle to Geological Time. — It 

 is, however, the feature of the theory advocated by Thiselton-Dyer 

 that this process of differentiation during the centrifugal dispersion 

 of plants from the north has affected not merely one migration from 

 the polar area, but those of all geological periods during which the 

 land-masses have preserved the main characters of their present 

 arrangement. Thus he would extend it to the Mesozoic Conifers, 

 and even suggests it in the case of the Glossopteris flora of Permo- 

 Carboniferous times. There is this to be said, however, in this last 

 connection, that we here open up questions relating to the Antarctic 

 continent, which Seward, in his recent monograph on the fossil 

 plants collected during the recent expeditions, believes to have been 

 the centre of the differentiation of the Glossopteris flora (see Brit. 

 Mus. publications, 1914). But the occurrence of this flora in the 

 Upper Palaeozoic beds of Russia and Siberia has been established, 

 and there is something to be said in support of Thiselton-Dyer' s 

 contention in his Philadelphia address that " an economy of hypo- 

 thesis is best served by assuming a northern origin and a dispersal 

 southward than by calling into existence a vast territory from the 

 Indian Ocean." 



The Views of some Australian Naturalists. — A view, which 

 is the very opposite of that advocated by Thiselton-Dyer is held by 

 some Australian naturalists, who consider that " the community of 

 austral life is explicable only by former radiation along land-routes 

 from the south polar regions." Hedley, whose important paper on 

 the palaeographical relations of Antarctica is here quoted (Linn. Soc. 

 Lond., June 1912), calls as witnesses two genera, Fagus and Arau- 

 caria, the distribution of which in the past and in the present is 

 usually regarded as indicating their home in the north. Fagus and 

 Araucaria, however, cannot be treated together in this connection, 

 the first named belonging to the age of the Angiosperms, the second 

 to the Mesozoic Conifers. Though Antarctica apparently does not 

 share in the history of the plant- world since the appearance of the 

 Dicotyledons in force in the Upper Cretaceous age, it took a part in 

 the earlier periods, and whilst prevented from figuring in the history 

 of the Angiosperms, it may have preserved a record of world-ranging 

 Conifers, 



Let us take the case of Araucaria. In his Philadelphia address 

 Thiselton-Dyer observes that if we go back to the Jurassic age, and 

 turn to Coniferse, the structures of which lend themselves to recog- 

 nition in the fossil state, " we find in Araucaria, a genus now repre- 

 sented by a few species in both divisions of the southern hemisphere, 

 abundant evidence that it was once widely dispersed in the northern." 

 In this connection one may remark that four species of Araucaria 

 are included in Knowlton's list (quoted by Harshberger, p. 176) of 

 Cretaceous and Tertiary plants of North America known up to 1898. 

 In the case of Fagus it will be sufficient to note that eleven species 

 are referred to it in the list of fossil North American plants just 

 mentioned. But the linking together of this genus with Araucaria 

 in support of the Antarctic hypothesis raises another point. One 

 might admit the presence of a Mesozoic flora of Conifers in the 



