6] 



FOUNDATIONS OF MODERN DISTRIBUTIONS 



157 



from the floristically poor regions lying immediately to the east, was 

 problematical to say the least, and from the west was wellnigh 

 impossible as the Atlantic was now indubitably in existence. On 

 the other hand, in North America and eastern Asia the warmth- 

 loving plants among others were free — and evidently often able — 

 to migrate far south in the lowlands or along mountain ranges, 

 subsequently to return when the ice retreated and suitable conditions 

 were reinstated in the North. Much the same appears to have 

 happened in the Balkan region. To such considerations is attributed 

 the presence of many living woody plants, such as members of the 

 Magnolia family (Magnoliaceae), in eastern North America and 



Fig. 42. — Known distribution of species of Liriodendron in Tertiary times (black) 

 and nowadays (hatched). (After Good). 



southeastern Asia but, on the other hand, their absence from Europe, 

 though their remains in fossil deposits indicate that they were once, 

 widely distributed and locally plentiful in all three of these continents 

 Fig. 42 shows the known distribution of Tulip-trees {Liriodendron 

 spp., Magnoliaceae) in Tertiary times (black) and living today 

 (hatched), and Fig. 43 shows the even more drastic restriction of 

 the giant Redwoods. Fig. 44 indicates the periods during which 

 the various parts of North America are supposed to have been free 

 from major ice-sheets. Even in the last of the interglacial periods, 

 many plants persisted much farther north than they grow todays 

 most notably in Europe — and it has been suggested that we have 

 still not emerged permanently from the Ice Age. 



Whereas it is thought that considerable expanses of territory in 



