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they have been placed, than that the respective phases 

 were produced in situ on patterns almost coincident. 



I have before announced my conviction, that generic 

 areas have a real existence in Nature's scheme; and 

 that, consequently, where species which are so intimately 

 allied that they can with difficulty be distinguished, 

 prevail, there is presumptive reason to suspect (until at 

 least the contrary is rendered probable) that the areas 

 which they now colonize were once connected by an 

 intervening land, or, in other words, that the migra- 

 tions of the latter were brought about, through ordinary 

 diffusive powers, from specific centres within a moderate 

 distance of each other. I say "presumptive reason/' 

 because there are undoubted exceptions to this law (as 

 to every other), and it can therefore be only judged of 

 on a broad scale. Still, I contend that in a wide sense 

 it holds good ; and that, consequently, if closely related 

 " species ' are traceable in countries which geology 

 demonstrates to have been far asunder during the entire 

 interval since the first appearance of the present animals 

 and plants upon our earth, there is at any rate an 

 a priori probability that they are no species at all, but 

 permanent geographical states, which have been slowly 

 matured since their casual introduction beyond their 

 legitimate bounds. 



If we except those forms which are in reality but 

 modifications, from climatal and other causes (and 

 which have, therefore, been wrongly quoted as distinct) ; 

 I believe that a vast proportion of the species which 



