part 2] 



KEVIEW OF PLIOCENE FLORAS. 



153 



The circuinpolar distribution of aquatic and sub-aquatic specios possibly 

 points to some special means of dispersal, perhaps migrating birds. The 

 Himalayan-European distribution of living West European, and of many 

 exotic, species, lends very strong confirmation to a suggestion made by us in 

 ' The Pliocene Floras.' It is worth quoting what we there stated (pp. 22, 

 23) : — ' The immense area of high land in Central Asia, including Tibet, the 

 Himalaya, and Western China, may have acted as a second centre for the 

 origin and dispersal of temperate species, which radiated from these uplands 

 when the climate became colder, just as they radiated from the shores of the 

 Arctic Sea. If such a dispersal as this took place it must have been in 

 Miocene or Pliocene times, when the Northern Hemisphere was cooling.' 



I have already pointed out that the curve indicates the middle of the 

 Miocene as the period at which the process of infiltration of the new flora 

 began, to continue throughout the whole of the Pliocene. 



It is clear, therefore, that the further work on Pliocene floras has strongly 

 confirmed our suggestion, that the immense upland area of Asia formed a 

 subsidiary centre of dispersal in the Pliocene ; and it seems very probable, 

 although I am not prepared to say that the suggestion is established, that 

 much of the living flora of the lowlands of Western Europe has been derived 

 from this source, by dispersal through the Near East, the Caucasus, and the 

 mountains of Southern and Central Europe, or by way of the Mediterranean. 



The incoming flora, which gradually supplanted the Chinese- 

 North American flora, from the middle of the Miocene onwards, 

 needs further analysis. 



To ascertain how its character changed throughout the Pliocene 

 Epoch, it is necessary in the first place to find its total percentage 

 at the different stages. These are given by the complementary 

 percentages to those of the Chinese-North American element. 

 Next, the percentages of the exotic element must be found by sub- 

 tracting the Chinese-North American percentage from those of 

 the total exotic flora. The following: table is then arrived at : — 



Table showing the Change in Character or the 

 Incoming Flora. 



Flora. 



Percentage of total 

 incoming flora. 



Percentage of exter- 

 minated element init. 



Cromerian 



Teglian 



Castle Eden 



Eeuverian 



96-26 



84 



69 



46 



36 



5 

 24 

 33 

 34 

 30 



If the figures of these two columns are then plotted as a curve 

 (fig. 2, p. 154) they show the general character of the change of 

 this incoming flora. In this curve the zero-point corresponds to 



m2 



