302 PHYSIOGNOMY OF PLANTS. 



gradual transformations of species, and suppose the different kinds 

 of parrots proper to two islands not far removed for 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 our 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 vegetation in each species, however different its duration 

 may be, requires, in order to be successfully passed through, a cer~ 

 tain minimum of temperature. (Playfair, in the Transactions of the 

 Royal Society of Edinburgh, vol. v. 1805, p. 202 ; Humboldt, on 

 the sum of the degrees of temperature required for the cycle of 

 vegetation in the Cerealia, in Mem. sur les lignes isothermes, p. 

 96; Boussingault, Economic rurale, t. ii. pp. 659, 663, and 667; 

 Alphonse Decandolle, sur les causes qui limitent les especes vege"tales, 

 1847, p. 8.) But all the conditions necessary for the existence of 

 a plant, either as diffused naturally or by cultivation conditions 

 of latitude or minimum distance from the pole, and of elevation or 

 maximum height above the level of the sea are farther complicated 

 by the difficulty of determining the commencement of the thermic 

 cycle of vegetation, and by the influence which the unequal distri- 

 bution of the same quantity of heat into groups of successive days 

 and nights exercises on the excitability, the progressive develop- 



