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obstruction, but a secondary part. Still, there is good 

 reason to believe that the ancient tract of which it is a 

 portion was broken up at a comparatively early date 

 after the creation of those peculiar organic forms which 

 found their birthplace within its bounds ; and that, con- 

 sequently, the latter could not have wandered far (if we 

 except those species on w r hich unusual powers of diffu- 

 sion were bestowed) when the land of passage began to 

 give way. Hence, even the sea, in this particular in- 

 stance, partakes almost of the character (no less than 

 the mountain heights) of an original impediment ; and 

 Madeira therefore may be safely quoted as an example 

 in which two barriers, of a primary nature, are united ; 

 and where, consequently, we may anticipate those ultra 

 phenomena of areal limitation upon which we have 

 been just commenting. 



But let us now inquire, whether the hypothesis at 

 which w T e have arrived will stand the test of experience ; 

 for unless it will do so, we might have been spared the 

 labour of propounding it. Madeira is a country com- 

 posed of narrow mountain ridges, which radiate from 

 central crests, and form the lateral boundaries of deep 

 and precipitous ravines. Modifications of this structural 

 type are of course traceable everywhere ; the upland 

 tracts are often undulating and broad, and the buttresses 

 which slope towards the sea are sometimes expansive 

 and irregular : yet upon the whole the above description 

 is correct, and we may accept it in a generic sense. 

 Now we may premise that, even to this day, it is an 



