348 cosMoa. 



tribution of heat over the surface of 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 to- 

 ward the poles, and of the relations in which, in different part? 

 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 

 characterized features of tropical scenery to direct my investi- 

 gations toward these subjects. 



The study of the geographical distribution of animals, re- 

 garding 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 

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

 more especially those of the isochimenal lines, correspond with 

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

 and of animals which do not wander far from their fixed hab- 

 itation, 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 affect- 

 ed 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 forward respecting the origin of the flora of 

 the British mountains be true — and every geological and botanical prob« 

 ability, so far as the area is concerned, favors it — then must we endeav- 

 or to find some more plausible cause than any yet shown for the pres- 

 ence 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 Hum- 

 boldt, the great organizer of the science of natural history geography, 

 demonstrated, that zones of elevation on mountains correspond to par 

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

 case might k-i. It is well known that this correspondence is recogniz- 

 ed in the general /ac£es 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 climatal 

 zones of animal and vegetable life are mutually repeated or represented 

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

 at all) distinguished between the evidence of that law, as exhibited by 

 representative species and by identical. In reality, the former essen- 

 tially depend on the law, the latter being an accident not necessarily 

 dependent upon it, and which has hitherto not been accounted for. In 

 the case of the Alpine flora of Britain, the evidence of the activity of 

 the lav» , and the inHuence of the accident, are inseparable, the law bo* 



