Chapter X — 173 — Historical Causes 



in temperature. Hence, there was created a basis for the presumption 

 that there probably had been at one time a center of species-formation 

 in the region around the South Pole, similar to that assumed for the 

 North Polar region, and a migration of floras, under the influence of 

 cHmatic changes, from south to north. Thus, the mono-boreal theory 

 of the origin of life was replaced by the theory that this role was 

 played by the lands encircling both poles. 



The theory of the polar origin of floras, in the light of our present 

 knowledge, cannot be accepted. It has now been established that 

 climatic zones have existed during the entire history of the earth and 

 that ice ages occurred not only in the Quaternary period but also dur- 

 ing other geological periods, the glaciated regions, moreover, not being 

 located in the present polar areas but in other parts of the globe. 



The assumption that there were only these polar centers of develop- 

 ment of floras is likewise contraverted by the fact that the existence of 

 other centers of species-formation has been definitely established. The 

 view advanced by Hallier (1912), Golenkin (1925), Irmscher (1922, 

 1929), and others that the tropics served as a center of origin of the 

 angiosperms, whence they at various times penetrated into temperate 

 regions, has much data to support it, provided it is accepted that there 

 took place shif tings of the tropical zone. 



An approach to the solution of all these enigmatic moments in the 

 history of the earth, making it seemingly impossible to explain the past 

 distribution of its floras, has been provided by the theory of conti- 

 nental drift. 



6. Theory of Continental Drift. — From the foregoing we have seen 

 that the two chief theories as to the past history of the earth's surface 

 — the theory of the permanence of the oceans and continents and the 

 theory of trans-oceanic land-bridges — are mutuaUy contradictory. A 

 solution of this riddle may be found, if one accepts the permanence not 

 of the separate oceans and continents as such but the permanence of 

 the relative area of land and sea taken as a whole. Then, by assum- 

 ing, as Wegener (1929) does in his theory of continental drift, the 

 possibility of a horizontal drifting of the continents, which, so to say, 

 float on an underlying viscous substratum (the so-called "sima", com- 

 posed of basic, igneous basalts), we are enabled, without departing 

 from the principle of the permanence of oceans and continents, to ex- 

 plain the existence of connections between the continents, not on the 

 assumption that there formerly existed additional continents where 

 there is now sea, but on the assumption that our present continents 

 were formerly in direct contact with one another. 



According to this hypothesis, it is assumed that as late as the end 

 of the Paleozoic era the continents were all united in one great conti- 

 nent, Pangaea, which only in the Mesozoic era begins to rift apart, two 

 meridian lines of rupture being formed. These lines of rupture between 

 Euro-Africa and America, on the one hand, and between Africa and 

 India, on the other, lead to the creation of the Atlantic and Indian 

 oceans. The separation of South America from Africa becomes wider 

 and wider during the Cretaceous period, but even at the beginning of 

 the Tertiary period there still persists a slight connection between the 

 northeastern coast of South America and the west-central coast of 



