38 



If we consider the existing distribution of plants upon the earth's surface, we 

 find that two classes of phenomena present themselves to our notice ; one which 

 evinces a close dependence upon the distribution of heat and other elements of 

 climate, and is simply the result of the peculiarities of constitution in the various 

 species, which, as they cease to meet with their climatal requirements, become 

 earlier or later arrested in their progress towards the equator or the poles ; the 

 other, clearly independent of physical causes, and remaining as a residuum of 

 unexplained facts after the influence of heat, moisture, and other elements of 

 climate, have been fully taken into account, and for the explanation of which we 

 must look beyond any causes at present in operation ; for example, when 

 we examine the lists of plants which grow spontaneously in different countries, we 

 find that, though these countries may be closely alike in physical condition, the 

 species, and even natural families which inhabit them, are more different in pro- 

 portion as these countries are widely removed from each other. Thus the vegeta- 

 tion of Europe has a tolerably uniform character ; but, if we pass over to America, 

 we find that a very small proportion of British plants grow wild there ; and, if we 

 extend our glance to Australia, we meet with but one or two European species, 

 the flora being wholly unlike that of the northern hemisphere, and made up in 

 great part of natural families which have no European representatives 

 (Goodeniads, Epacrids, Stylidacae). That the absence of the requisite physical 

 conditions is not the cause of this, we have abundant proof in the fact that so 

 many of our British plants have readily established themselves when once intro- 

 duced to America ; and conversely, that American species have been thoroughly 

 naturalised in Britain and other parts of Europe. To explain these facts then, 

 no other course is open but hypothesis ; but no hypothesis can be fully received 

 until we have made certain that the facts on which it is based are not the results 

 of physical causes, unless also it is found to explain all the facts when thus 

 sifted. 



These striking differences in the floras (as they are termed) of distant regions 

 may, it has been found, be expressed on maps by marking down the regions of 

 the world in which the various families have their greatest development. Thus 

 in the tropics we have the region of palms and bananas, &c. ; and in temperate 

 latitudes, the region of oaks and European dicotyledonous trees, with others, 

 which I must not stay to mention. Now the hypothesis which the study of these 

 phenomena has induced botanical geographers most generally to adopt is that 

 which supposes species or groups of species to have been created at different 

 centres, from which, by their gradual spreading, they have, like so many inter- 

 secting circles, populated the intervening regions, and produced the complex 

 character of their existing floras. 



If we turn our attention to the vegetation of Great Britain, and compare it 

 with that of surrounding countries, we find that the great mass of species distri- 

 buted through the lowlands are identical with those of what is termed the north 

 German or Central European Flora ; that in the south western extremity of 

 England there is an addition of species more peculiar to Prance; while in theS.W. 

 of Ireland, together with these, occur several which, on the continent of Europe, 



