INTRODUCTORY ESSAY. xvn 



•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 is 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 climate of the south 

 temperate zone. I have also shown how conspicuously this kind of climate influences 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, Rosa, Pinus, 

 Prunus, Camellia, Rubus, Kadsura, Fragaria, iEsculus, 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 element 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 B»s-i5* 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 1 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 povjer 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- 



d 



