6] FOUNDATIONS OF MODERN DISTRIBUTIONS 165 



the plants were able to get away from the competition of ranker 

 (but often less hardy) types. 



Altogether it seems most reasonable to consider the post-Pleisto- 

 cene flora of those territories of the northern hemisphere which 

 were freed from the last major glaciation as being made up of : 

 (i) some elements which had with little doubt persisted in at least 

 recent unglaciated ' refugia ', particularly in sheltered coastal areas, 

 (2) elements spreading from sheltered refuges afi^orded particularly 

 by mountain systems where the Ice Age did not have such catastrophic 

 consequences as to destroy the characteristic Tertiary flora even if 

 it did cause impoverishment, (3) probably numerous elements which 

 migrated northwards from the territories bordering the southern 

 extremity of the ice after it retreated, and (4) recent immigrants 

 from afar (or at least from regions not drastically affected by the 

 Pleistocene), whose establishment has been favoured, in areas recently 

 freed from glaciation, by the local lack of competition from already 

 closed {i.e. continuous) vegetation. These recent migrants were 

 aided by natural means — wind, water, or animals — or by Man, 

 through intentional or accidental importation, and many such migra- 

 tions are still going on plentifully all the time. Examples are afl"orded 

 by the considerable numbers of weeds that have recently become 

 established in ' open ' areas, including inhabited parts of West 

 Greenland, and the westward advance of Asian species into Europe. 



Continental Drift, Shifting Poles, Land-Bridges, etc. 



The present distribution of any given plant species is in part a 

 reflection of the geological revolutions and climatic changes that 

 have occurred in the world during the period of its existence as a 

 species. In former sections of this chapter we considered the 

 efl^ects of climatic change in the past and particularly the significance 

 of the Pleistocene Ice Age. It is now time to examine some other 

 leading theories that have been advanced in an attempt to explain 

 the current distribution of particular plants. 



Perhaps the most promising and plausible (though still highly 

 controversial) theory in this connection is that of ' continental drift ', 

 which is often identified with the name of its principal proponent 

 of recent times, the late Dr. Alfred Wegener. This ' displacement 

 hypothesis ' is based on the assumption that the present-day con- 

 tinents once formed part of a single land-mass (Pangaea), or, accord- 

 ing to a recent modification, two land-masses, whose continents 



