VII. LONGITUDE. 447 



ther southward than its own latitude. And as we go 

 northward the cessation points of species are attained 

 and left behind in rapid succession. But no botanist will 

 sufficiently understand the botany of Britam, and the 

 relations of its flora in the general system, without a fair 

 knowledge of European botany between latitudes 40 and 

 70, and some small knowledge about that of Asia and 

 America in latitudes nearly corresponding therewith, or 

 somewhat more boreal. 



If we should trace round the globe within any given 

 range of boreal latitude, whether that of Britain itself or 

 some other not far distant, we should find a change in 

 the flora gradually increasing as we receded from our 

 starting point, and thus increased our longitudinal dis- 

 tance. Species after species would disappear ; and their 

 vacated place in the flora would become occupied hy other 

 species, successively appearing, and serving as their sub- 

 stitutes or representatives. But great as the total dif- 

 ference eventually becomes, and numerous as the sub- 

 stituted species are found to be, it seems impossible to 

 point out any line or parallel of longitude ; and to say 

 that there in particular the floral change occurs. Moun- 

 tain masses and oceanic spaces may no doubt make the 

 changes appear more abrupt ; but in such cases it is of 

 course to these other conditions of physical geography, 

 and not to longitude simply, that we must look for the 

 related explanations. 



Less of the changes alluded to are observable in very 

 high latitudes, where the longitudinal circumference of 

 the globe becomes much shortened under equal latitude ; 

 where the climate becomes more similar under difl"erent 

 longitudes ; and where the polar centre may be held a 

 common vanishing point of distribution. But as we pass 

 from arctic into more temperate latitudes those conditions 



