E. V. Wulff —172— Historical Plant Geography 



Simroth; this symmetry, however, may be ascribed to other causes, 

 as we shall show below. 



5. Theory of the Polar Origin of Floras. — Another attempt to avoid 

 the need of trans-oceanic land-bridges to explain the present distribu- 

 tion of floras is found in the theory of the origin of the latter from a 

 single center lying in the north polar region, whence they spread radi- 

 ally toward the south in three directions — through Europe into Africa, 

 through Asia and Malaysia into Austraha, which were at that time 

 connected, and through North America into South America. As a 

 basis for this theory there served the investigations of Heer (1868) of 

 the fossil flora of the Arctic. 



Heer, in a number of papers, later collected in his "Flora Arctica", 

 pointed out that in former geological epochs there grew in the Arctic 

 woody plants now found only in temperate or subtropical regions. His 

 data showed that climatic conditions in the Arctic during the Tertiary 

 period were entirely different from now and supposedly confirmed the 

 view that the polar region was the initial center of origin of floras. 



This point of view was first advanced by Forbes and was later 

 developed by Darwin, Berry, and others. It is based on data indi- 

 cating that beginning with the Tertiary period the floras and faunas 

 succeed one another in such a way that each successive one forces its 

 predecessor to the south. This shifting of floras and faunas pre- 

 sumably began from the moment of the differentiation into climatic 

 zones, which, according to the views then held, did not take place until 

 the end of the Cretaceous or the beginning of the Tertiary period, prior 

 to which the climatic conditions of the earth were allegedly uniform. 

 Consequently, the lowering of temperature conditions, which reached 

 its apogee during the Ice Age, must have had its effect on the vege- 

 tation of the entire globe and must have induced a migration of floras 

 from north to south. On the basis of the foregoing, the proponents 

 of this theory held that the flora that had inhabited the body of land 

 encircling the North Pole, which body of land prior to the Ice Age 

 was presumably even larger and more compact than now, constituted 

 the initial flora from which arose all the vegetation at present inhabit- 

 ing the earth, and that the steady decrease in temperature in the polar 

 regions forced the vegetation ever farther and farther south (Fiirsten- 

 BERG, 1909). 



In order to explain the similarities in the floras of South America, 

 Australia, and South Africa, and the afi&rmations of Hooker as to the 

 circumpolar nature of the antarctic flora, it was presumed that there 

 was a trans-equatorial migration of the flora of the northern hemi- 

 sphere into the southern, although Darwin and Hooker had them- 

 selves suggested that in the past there might have existed an antarctic 

 continent embracing what are at present separate islands and the ex- 

 tremities of the continents of the southern hemisphere. Subsequently, 

 the existence of such a continent was generally accepted, the remains 

 of fossil flora found within the Hmits of present-day Antarctica having 

 confirmed not only that the lands of the southern hemisphere had 

 formerly been connected with one another by way of this Antarctic 

 continent but also that there had occurred a change of climatic con- 

 ditions in the Antarctic just as in the Arctic, in the sense of a decrease 



