XVlll FLORA OF TASMANIA. 
perate regions, and of the temperate species of both hemispheres on the mountains of intermediate 
tropical latitudes. 
On the other hand, we have sufficient evidence of many of what are now the most tropical 
Orders of plants having inhabited the north temperate zone before the glacial epoch ; and it is diffi- 
cult to conceive how these Orders could have survived so great a reduction of the temperature of the 
globe as should have allowed the preglacial temperate Flora to cross the Equator in any longi- 
tude. It is evident that, under such cold, the most tropical Orders must have perished, and their 
re-creation after the glacial epoch is an inadmissible hypothesis* 
29. It remains then to examine whether, supposing the glacial epochs of the northern and 
southern hemispheres to have been contemporaneous, the relations of land and sea may not have been 
such as that a certain meridian may have retained a tropical temperature near the Equator, and thus 
have preserved the tropical forms. Such conditions might perhaps be attained by supposing two 
large masses of land at either pole, which should contract and join towards the Equator, forming 
one meridional continent, while one equatorial mass of land should be placed at the opposite meridian. 
If the former continent were traversed by a meridional chain of mountains, and so disposed that the 
polar oceanic currents should sweep towards the Equator for many degrees along both its shores, its 
equatorial climate would be throughout far more temperate than that of the opposite equatorial mass 
of land, whose climate would be tropical, insular, and humid. 
30. The hypothesis of former mountain chains having afforded to plants the means of migration, 
by connecting countries now isolated by seas or desert plains, is derived from the evidence afforded 
by geology of the extraordinary mutation in elevation that the earth's surface has experienced since 
the appearance of existing forms of animals and plants. In the Antarctic Flora I suggested as an 
hypothesis that the presence of so many Arctic-American plants in Antarctic America might be 
accounted for by supposing that the now depressed portions of the Andean chain had, at a former 
period, been so elevated that the species in question had passed along it from the north to the south 
temperate zone ;f and there are some facts in the distribution of species common to the mountain 
Floras of the Himalaya and Malay Islands, and of Australia and Japan, that would well accommo- 
date themselves to a similar hypothesis. Of such submerged meridional lands we have some slender 
* The question of the state of the mean temperature of the globe during comparatively recent geological periods 
is yearly deriving greater importance in relation to the problem of distribution. Upon this point geologists are not 
altogether clear, nor at one with the masters of physical science. Lyell (Principles, ed. ix. chap, vii.) attributes the 
; i to such a disposition of land and sea as would sufficiently cool the temperate zones; and he implies 
that this involves or necessitates a lowering of the mean temperature of the whole globe. Another hypothesis is, 
that there was a lowering of the mean temperature of the globe wholly independent of any material change in the' 
present relations of sea and land, which cold induced the glacial epoch. A third theory is that such a redispo- 
sition of land and sea as would induce a glacial epoch in our hemisphere need not be great, nor necessitate a 
decrement of the mean temperature of the whole earth. 
f The continuous extension of so many species along the Cordillera (of which detailed evidence is given in the 
Antarctic Flora) from the Tiocky a most remarkable fact, considering how great the break 
is between the Andes of New Granada and those of Mexico, and that the inierm , r but few 
ces for alpine plants. That this depression of the chain has had a powerful effeel in either limiting 
the extension of species which have appeared since its occurrence, or in i.ul.iciiiir .-l.;.!! sol clhnat, whiel, i , 
nee common to the north and south, is evidenced by the fact that a number of Fuegian 
i extend northward as alpines to the very shores of the Gulf of Mexico, but do not inhabit 
south to the Mexican Andes, but do not cross the inter- 
mediate depression and reappear in the Bolivian Andes. 
