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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 which 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 we 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 



