XVI 



being encased in a sort of leathery membrane, preventive, 

 to a considerable extent, of exhalation, retain for a long 

 enough period within their pulpy and distorted stems the 

 moisture which they had once imbibed, resembling certain 

 tardigrade animals in the slowness with which they execute 

 their vital functions, and like them exhibiting a tenacity of 

 life as remarkable as is the languor of their circulation. 



It would be an instructive, as well as an interesting 

 occupation, to trace in detail the effect of this and other 

 climatic conditions upon the several plants which occur in 

 particular regions or countries ; but after having exhausted 

 all our ingenuity in referring their range and distribution 

 to the influence of one or more of such causes, much, it may 

 be apprehended, will remain unaccounted for, and an ample 

 margin left for further speculation. It is difficult, for in- 

 stance, by any such considerations to explain why all the 

 Heaths, excepting five or six European species, come from 

 the Cape ; whilst the Epacrises, a nearly allied family, are 

 confined to Australia ; why the Orange tribe is derived ex- 

 clusively* from China or India ; the whole of one particular 

 division of syngenesious plants those which are called bi- 



* Unless Citrus spinosissima, from Guiana, and Limonia ambigua, from 

 Florida, be genuine species. 



