26 PLANT GEOGRAPHY 



mountain-chains, such as the American Cordilleras, the 

 Cameroons, or the Malaysian axis, northern plants may 

 have crossed the torrid zone to descend in the southern 

 hemisphere, and such migrations may have long ante- 

 dated the Glacial Period. In this way the Scandinavian 

 flora seems to have reached into every latitude, extending, 

 perhaps, by way of a chain of " sub-antarctic " islands 

 to Tasmania and New Zealand. Other important 

 elements in the New Zealand flora seem to have traversed 

 a Miocene land connection from New Guinea. 



With the gradual disappearance of Glacial conditions, 

 migration may be presumed to have taken place in 

 reversed directions. As there is every reason to believe 

 that the whole of Scandinavia, of the British Isles, and 

 of Canada were glaciated, the whole of the existing floras 

 of those countries must be the result of such post-Glacial 

 return migrations. As there is evidence that the pro- 

 longed Glacial cold was intermitted by milder periods, 

 so it has been argued that it was followed by alternations 

 at l^ist in north-west Europe of rainy " insular " 

 periods and drier " continental " ones. During the 

 former, for instance, bog-moss accumulated as peat; 

 during the latter pine forests might spring up on the 

 dried-up bogs. During these mild " insular " periods, 

 favoured by warm ocean currents from equatorial 

 latitudes, the so-called Atlantic flora an assemblage of 

 plants, of African parentage, unable to withstand such 

 severe winter cold as occurs in the inland regions of 

 temperate continents seems to have spread from 

 Cameroons or Atlas, by way of the coasts of Portugal or 

 Brittany, to Ireland, Cornwall, or the south-west coast 

 of Norway. 



Thus only, in the course of long wanderings to and 

 fro, have the floras of different lands attained to the 

 proportions in which their species now exist. Apparently 

 Cretaceous in origin, they have been added to and 

 reduced, and in many cases modified specifically or 

 varietally by tropical heat and glacial cold, by super- 

 abundant moisture, and by drought. 



