INTRODUCTORY ESSAY. xvii 
range of species into regions characterized by what would otherwise be to them destructive tempera- 
tures. 
I have, in the ' Antarctic Flora/ shown that the distribution of tropical forms ia extended 
into cold regions that are humid and equable further than into such as are dry and excessive; 
and, conversely, that temperate forms advance much further into humid and equable tropical regions 
than into dry and excessive ones; and I have attributed the extension of Tree-ferns, Epiphytal 
Orchids, Myrtacese, etc., into high southern latitudes, to the moist and equable- eliiuate of the south 
temperate zone. I have also shown how conspicuously this kind of climate inlluenees the distribu- 
tion of mountain plants in India, where tropical forms of Laurel, Fig, Bamboo, and many other 
genera, ascend the humid extratropical mountains of Eastern Bengal and Sikkim to fully 9000 
feet elevation ; and temperate genera, and in some cases species, of Quercus, Salix, Itosa, Pinus, 
Prunus, Camellia, Rubus, Kadsura, Fragaria, ^Esculus, etc., descend the mountains even to the level 
of the sea, in lat. 25°. In a tropical climate the combined effects of an equable climate and 
humidity in thus extending the distribution of species, often amount to 5000 feet in elevation or 
depression (equivalent to 15° Fahr. of isothermals in latitude), a most important clement in our 
speculations on the comparative range of species under existing or past conditions; and when to 
this is added that the average range in altitude of each Himalayan tropical and temperate and alpine 
species of Flowering Plant is 4000 feet, which is equivalent to 12° of isothermals of latitude, we can 
understand how an elevation of a very few thousand feet might, under certain climatic conditions, 
suffice to extend the range of an otherwise local species over at least 25° parallels of latitude, and 
how a proportionally small increase of elevation in a meridional chain where it crosses the Equator, 
may enable temperate plants to effect an easy passage from one temperate zone to the other. 
27. To explain more fully the present distribution of species and genera in area, I have recourse 
to those arguments which are developed in the Introductory Essay to the New Zealand Flora, and 
which rest on geological evidence, originally established by Sir Charles Lyell, that certain species 
of animals have survived great relative changes of sea and land. This doctrine, which I in 
that Essay endeavoured to expand by a study of the distribution of existing Southern species, 
has, I venture to think, acquired additional weight since then, from the facts I shall bring forward 
under the next head of Geological Distribution, and which seem to indicate that many existing 
Orders and Genera of plants of the highest development may have flourished during the Eocene 
and Cretaceous periods, and have hence survived complete revolutions in the temperature and geo- 
graphy of the middle and temperate latitudes of the globe. 
28. Mr. Darwin has greatly extended in another direction these views of the antiquity of many 
European species, and their power of retaining their fades unchanged during most extensive migra- 
tions, by his theory of the simultaneous extension of the glacial temperature in both hemispheres, and 
its consequent effect in cooling the tropical zone. He argues that, under such a cold condition of the 
surface of the globe, the temperate plants of both hemispheres may have been almost confined to the 
tropical zone, whence afterwards, owing to an increment of temperature, they would be driven up the 
mountains of the tropics, and back again to those higher temperate latitudes where we now find most 
of them. I have already (New Zealand Essay) availed myself of the hypothesis of an austral glacial 
period, to account for Antarctic species being found on the alps of Australia, Tasmania, and New 
Zealand; and if as complete evidence of such a proportionally cooled state of the intertropical 
regions were forthcoming as there is of a glacial condition of the temperate zones, it would amply 
suffice to account for the presence of European and Arctic species in the Antarctic and south tern- 
