ae 
72 [ 300] 
is exclusively confined to America : Calathea, . genus of Marantacew, is 
only found on the same continent: cinnaino n, cloves, and nutmegs are 
confined to the Indian Archipelago ; and hundreds of other instances are: 
to be named of similar exclusive stations. Whether these differences 
depend upon geological causes; or arise from Some other circumstances, is 
entirely unknown. 
Such are the most striking facts connected with the distribution of tem- 
perature with respect to vecetation. {t will have been seen that little is 
nown of the proportion of humidity in’ the atmosphere of different: eli- 
mates, ‘and that the amount of light in various latitudes has scarcely been 
noticed. * That the effect of both these agents upon vegetation is most 
I shall now pro oceed to state what is known or conjectured of the distri- 
bution of the different orders or divisions of vegetables over the surface of 
naire des Sciences Naturelles, voli xviii. p. 422; in high is <oniiehenaine 
the sum of all ‘that is known of the laws that are observed in the distri- 
bution of the various forms of vegetation. « The numerical relations of 
the forms of vegetation are capable of being investigated in two very differ: 
tion? What propor rtion is borne by Moh dentyledoess ae Picotyleleces 
» Questions of this kind refer rather to the science of vegetable organization . 
and of mutual affinities. But if, instead of stand ying. saniaae groups 0 
tions they bear to climate or to the distribution over the em of the 
a —_ questions of a much more varied nature will arise. Which 
mstance, are more predominant i - the torrid one thi in vat 
eel Are Com sara ws more numerous in the ame Ene lel of 
at rt 
pole rate similar law of decrement in rising from the pane into 
the mountains of the sathior? Do the proportions borne by one poss 
another vary on the: same. isothermal line; and are such proportions the 
‘Same on either side of the equato; vas bates hese are, st ‘peas — 
ae 
_ tions of geographical’ botany : they. connected with the n 
problems of met meteorology, and of tits Secs of the globe in genera 
“ In studying the: geographical distribution of particular erp we can 
ioe -either’at a consideration of particular species, genera, or natural _ 
families. It often’ happens that a particular species, especially of those: , 
"ea which I have called social, covers a vast extent of f country: such, for 
xe, eis in — acai = heaths and forests‘of pines; such are, in 
semblages of multitades of Cactus, Croton, 
i and B Brat, é; the same species. i is curious to examine such 
