488 
ISLAND LIFE. 
[part II. 
period were so much like existing species that although 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 
living . 1 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. 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 Migration 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. Ettinghausen 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 Ileer, presents a still larger proportion of living 
genera. 
