520 ISLAND LIFE 



the plants of the Miocene period of Europe were so much 

 like existing species that although they have generally re- 

 ceived fresh names they may well have been identical ; 

 and a large jDroportion of the vegetation during the whole 

 Tertiary period consisted of genera which are still living.^ 

 But from what is now known of the rate of sub-aerial 

 denudation, we are sure, that during each division of this 

 period many mountain chains must have been considerably 

 lowered, while we know that some of the existing ranges 

 have been greatly elevated. xA.ncient volcanoes, too, have 

 been destroyed by denudation, and new ones have been built 

 up, so that w^e may be quite sure that ample means for the 

 transmission of temperate plants across the tropics, may 

 have existed in countries where they are now no longer to be 

 found. The great mountain masses of Guiana and Brazil, 

 for example, must have been far more lofty before the 

 sedimentary covering was denuded from their granitic 

 bosses and metamorphic peaks, and may have aided the 

 southern migration of plants before the final elevation of 

 the Andes. And if Africa presents us with an example of 

 a continent of vast antiquity, we may be sure that its 

 great central plateaux once bore far loftier mountain 

 ranges before they were reduced to their present condition 

 by long ages of denudation. 



Proofs of Migration ly Way of the Andes. — We are now 

 prepared to apply the principles above laid down to the 

 exj^lanation of the character and affinities of the various 

 portions of the north temperate flora in the southern 

 hemisphere, and especially in Australia and New Zealand. 



At the present time the only unbroken chain of 

 highlands and mountains connecting the Arctic and north 

 temperate with the Antarctic lands is to be found in the 

 American continent, the only break of importance being 

 the comparatively low Isthmus of Panama, where there is 



^ Out of forty-two genera from the Eocene of Sheppey enumerated 

 by Dr. Ettingshausen in the Geological Magazine for January 1880, only 

 two or three appear to be extinct, while there is a most extraordinary inter- 

 mixture of tropical and temperate forms — Musa, Nipa, and Victoria, with 

 Corylus, Prunus, Acer, &c. The rich Miocene flora of Switzerland, 

 described by Professor Heer, presents a still larger proportion of living 

 •iencra. 



