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features, it may be asked, which the D. obscurcguttatus has 

 adopted, since its first arrival from more northern latitudes 

 over an unbroken* continent ? It has not altered much, 

 after all : it is, however, the nature of the alterations, 

 and their constancy, which give them their real import- 

 ance. In a few words then, the insect is rather larger 

 and more robust than its European analogue, and (to 

 omit other minor differences) its wings are evanescent. 

 But this, on our above hypothesis, is precisely what we 

 should have expected : for, since it is self-evident that 

 the species cannot have been naturalized accidentally on 

 these mountains, and since geology informs us that a 

 vast interval has elapsed since the Madeiran islands were 

 portions of a continuous whole, we have at once a suffi- 

 cient time assured us for the modifications to be com- 

 pleted, and to appear at length permanently adjusted in 

 accordance with the conditions and influences which 

 locally prevail. 



There are other examples which might be quoted in 

 support of my theory, that isolation, when involving a 

 sufficient period of time, has a direct tendency either to 



* I do not think it necessary to apologize for the apparent dis- 

 posal of this qucestio vexata ; because, from the wildness of the 

 upland ridges to which the D. obscuroguttatus is in Madeira ex- 

 clusively confined, I deem it an absolute impossibility that it could 

 ever have been introduced, through any chance agencies whatsoever. 

 And hence, unless we reject the doctrine of specific centres in toto, 

 I contend that it must have migrated, together with other insects 

 similarly circumstanced, by ordinary means, and without natural 

 impediments, from its own area of diffusion. 



