112 PHYSIOGNOMY OF PLA.XTS. 



This agreement in the number of species in each family 

 compared to the whole number of phaenogamous species in 

 the Floras of Trance and Germany, would not by any means 

 exist if the German species which are missing in France 

 were not replaced there by other types belonging to the 

 same families. Those who are fond of imagining gradual 

 transformations of species, and suppose the different kinds 

 of parrots proper to two islands not far removed from each 

 other to present examples of such a change, will be inclined 

 to attribute the remarkable similarity between the two columns 

 of figures which have just been given, to a migration of 

 species, which, having been the same at first, have been altered 

 gradually by the long-continued action of climatic causes 

 during thousands of years, so that their identity being lost 

 they appear to replace each other. But why is it that ciur 

 common heather (Calluna vulgaris), why is it that our oaks 

 have never advanced to the eastward of the Ural Mountains, 

 and so passed from Europe to Northern Asia ? Why is there 

 no species of the genus Rosa in the Southern Hemisphere, 

 and why are there scarcely any Calceolarias in the Northern 

 Hemisphere ? The necessary conditions of temperature are 

 insufficient to explain this. Thermic relations alone cannot, 

 any more than the hypothesis of migrations of plants 

 radiating from certain central points, explain the present 

 distribution of fixed organic forms. Thermic relations 

 are hardly sufficient to explain the limits beyond which 

 individual species do not pass, either in latitude towards 

 the pole at the level of the sea, or in vertical elevation 

 towards the summits of mountains. The cycle of vegeta- 

 tion in each species, however different its duration, may be 



