OE )GRAPHY OF PLANTS. 357 



bution of heat over the surface ot the earth, and when the 

 arrangement of vegetable forms in natural families admitted 

 of a numerical estimate being made of the different forms 

 which increase or decrease as we recede from the equator 

 towards the poles, and of the relations in which, in different 

 parts of the earth, each family stood with reference to the 

 whole mass of phanerogamic indigenous plants of the same 

 region. I consider it a happy circumstance, that at the time 

 during which I devoted my attention almost exclusively to 

 botanical pursuits, I was led by the aspect of the grand and 

 strongly characterised features of tropical scenery, to direct 

 my investigations towards these subjects. 



The study of the geographical distribution of animals, 

 regarding which Buffon first advanced general, and in most 

 instances very correct views, has been considerably aided in 

 its advance by the progress made in modern times in the geo- 

 graphy of plants. The curves of the isothermal lines, and 

 more especially those of the isochimenal lines, correspo d with 

 the limits which are seldom passed by certain species of plants, 

 and of animals which do not wander far from their fixed 

 habitation, either with respect to elevation or latitude.* The 



* [The following valuable remarks by Professor Forbes, on the cor- 

 respondence existing between the distribution of existing faunas and 

 floras of the British Islands, and the geological changes that have 

 affected their area, will be read with much interest ; they have been 

 copied, by the author's permission, from the Survey Report, p. 16 : 



" If the view I have put fonvard respecting the origin of the flora of 

 the British mountains be true and every geological and botanical pro- 

 bability, so far as the area is concerned, favours it then must we endea- 

 vour to find some more plausible cause than any yet shown, for the 

 presence of numerous species of plants, and of some animals, on the 

 higher parts of Alpine ranges in Europe and Asia, specifically identical 

 with animals and plants indigenous in regions very far north, and not 

 found in the intermediate lowlands. Tournefort first remarked, and 

 Humboldt, the great organizer of the science of natural history geogra- 

 phy, demonstrated, that zones of elevation on mountains correspond to 

 parallels of latitude, the higher with the more northern or southern, as 

 the case might be. It is well known that this correspondence is recog- 

 nized in the general fades of the flora and fauna, dependent on generic 

 correspondences, specific representatives, and in some cases, specific 

 identities. But when announcing and illustrating the law, that climat J 

 zones of animal and vegetable life are mutually repeated or represented 

 by elevation and latitude, naturalists have not hitherto sufficiently (if n 

 ill) distinguished between the eyidence of that law, as exhibited by 



