ISLAND LIFE. 



[faet II. 



period were so mucli like existing species that althougli they 

 have generally received fresh names they may well have been 

 identical ; and a large proportion of the vegetation during the 

 whole Tertiary period consisted of genera which are still 

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

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

 period many mouatain chains must have been considerably 

 lowered, while we know that some of the existing ranges have 

 been greatly elevated. Ancient volcanoes, too, have been de- 

 stroyed by denudation, and new ones have been built up, so 

 that we may be quite sure that ample means for the transmis- 

 sion 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 ere they were reduced to their present condition by 

 long ages of denudation. 



Proofs of Mir/ration by way of the Andes. — We are now 

 prepared to apply the principles above laid down to the ex- 

 planation 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 



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

 by Dr. Ettingliausen 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 

 genera. 



