6] FOUNDATIONS OF MODERN DISTRIBUTIONS 171 



continental drift involving the ' like ' areas being once together but 

 subsequently separated ! 



It seems, however, that the theory of land-bridges is justified to 

 the extent that such phenomena come and go nowadays on a small 

 scale — for example with the emergence and submergence of isthmuses 

 with changes in the relative level of water, and with the throwing 

 up or destruction of beaches by the sea. Also, there can be little 

 doubt that such ' bridges ' have existed on a bigger scale in the past, 

 at least to the extent of once linking together some present-day 

 islands with their adjacent mainland over an area of continental 

 shelf, and joining in continuity such close land-masses as Alaska 

 and easternmost Siberia. Nor, according to some authorities, can 

 certain distribution-patterns in the southern hemisphere well be 

 explained without the supposition of an Antarctic land-bridge. 

 This may have joined South America to Australasia, to which two 

 regions such restricted genera as Nothofagus (Southern Beeches) and 

 Fitzroya are common, while also occurring as fossils on the Antarctic 

 Continent. 



The theory of permanence of oceans and continents, which grew 

 out of objections to, and largely opposes, those of land-bridges and 

 continental drift, presupposes the land-masses to have occupied their 

 present positions from pre-Cambrian times. However, it still leaves 

 unexplained many biogeographical phenomena — particularly those 

 that suggest the continents to have been connected at some period 

 — and is unable to cope with the high-arctic flourishing of luxuriant 

 vegetation in earlier ages. For although it has been suggested, and 

 may yet be maintained, that plants might have changed their climatic 

 requirements in the past, they can scarcely have done this sufficiently 

 to account for such extreme cases. 



This brings us to the theory of the polar origin of floras, which 

 in one or another form has some eminent advocates to this day. 

 When it was believed that climatic conditions were uniform through- 

 out the world in early geological ages, with diflerentiation into 

 climatic zones not taking place until the end of the Cretaceous or 

 beginning of the Tertiarv, it was widely thought that the floras of 

 the world spread rapidly from a single centre lying in the north 

 polar region — through Europe into Africa, through eastern Asia into 

 Malaysia and Australia, and through North America into South 

 America. This was the arctic or monoboreal theory of the origin 

 of floras. Subsequently such discoveries as that of fossil floras in 

 Antarctica indicated that a similarly drastic reduction in temperature 



