THE EVOLUTION OF THE PLANT- WORLD 25 



those of the Western Mediterranean, the South Atlantic, 

 and the Western Pacific, may have occurred in later, i.e. 

 Tertiary times. These, and the fact that Eocene and 

 Oligocene rocks are crumpled up, together with Jurassic 

 and Cretaceous ones, into the great fold-mountain axes 

 of our continents between the more primitive earth- 

 masses, make it difficult to think of any land-distribution 

 much like that of to-day before Miocene times. On the 

 other hand, by the close of the Pliocene period we may 

 have had oceans, continents, and a distribution of the 

 main groups of plants in very much the same proportions 

 as now. 



As the cold of the Glacial period came on, it seems to 

 have driven heat-loving or " tropical " plants south- 

 ward, followed by those of warm-temperate, cold- 

 temperate, and polar latitudes. 



" We may suppose that the organisms which now live under 

 latitude 60, lived during the Pliocene period further north under 

 the Polar Circle, in latitude 66-67. Now, if we look at a terrestrial 

 globe, we see under the Polar Circle that there is almost continuous 

 land from western Europe, through Siberia, to eastern America. 

 And this continuity of the circumpolar land, with the consequent 

 freedom under a more favourable climate for intermigration, will 

 account for the supposed uniformity of the sub- arctic and tem- 

 perate productions of the Old and New Worlds, at a period anterior 

 to the Glacial epoch " (Darwin, Origin of Species, p. 333). 



As such ferns as Matonia and Dipteris in the Malay 

 Peninsula are survivals of a still earlier flora, so 

 Liriodendron and Sassafras in China and the Southern 

 United States are survivals of the Cretaceous flora; and 

 Ramondia and Dioscorea pyrenaica Bub., representing the 

 tropical Gesneracece and the Yam family in the Pyrenees, 

 and Myrtus communis L., Laurus nobilis L., Ficus 

 Carica L., and Chamcsrops humilis L., in the Mediter- 

 ranean area, are sole survivors of their several orders 

 from pre-Glacial times. Many genera which then 

 flourished in Europe, and still grow in eastern Asia and 

 in eastern North America 



" were exterminated during the Glacial period, being cut off from 

 a southern migration, first by the Alps, and then by the Mediter- 

 ranean ; whereas in eastern America and Asia the mountain-chains 

 run in a north and south direction, and there is nothing to prevent 

 the flora from having been preserved by a southern migration into 

 a milder region " (A. R. Wallace, Island Life, ed. ii. p. 123). 



By ascending to greater altitudes on meridional 



