XX11 



the lower portions have been submerged ; he is at the same 

 time compelled to give a greater latitude than is generally 

 admitted to the variations produced by external conditions 

 upon the structure of plants, in order to account for the 

 distribution of allied, although not identical forms, over the 

 different parts of this once continuous area. 



The wider indeed the range may be over which a plant 

 is disseminated, the greater will be the number and force of 

 those external causes which tend to produce diversities of 

 form ; so that we have no right to argue, because within 

 the narrow limits of these islands species usually present 

 a near approach to uniformity, therefore that plants more 

 widely differing one from the other than any of our recog- 

 nized varieties, may not, if taken from the extreme limits 

 of the area in question, belong to the same species. If we 

 suppose a plant to have spread itself over a tract of land so 

 extensive, that the several portions of it may possess widely 

 different climates, it is quite possible to conceive such an 

 alteration to have been brought about in its structure, that 

 we might be disinclined to place together the extremes with- 

 out having before us the intermediate gradations in form 

 superinduced. 



After the submergence however of a continent, many of 



