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more than a fancy or romance, it is certain that nume- 

 rous members of many geographical assemblages must 

 have perished for ever during the gigantic sinkings 

 which have at various epochs been brought about. From 

 which it follows, that those groups, or clusters, of which 

 but few representatives {comparatively) are extant, will 

 be more or less abruptly terminated, according as the 

 original type to which they severally belong was peculiar, 

 and in proportion as the number of its exponents has been 

 reduced. 



Although there are many means through which 

 species may become annihilated, yet, since the sub- 

 sidence of a tract into the sea involves the maximum, of 

 loss which a space of that magnitude can sustain, the 

 above conclusion gives rise to a corollary : that it is in 

 islands that we should mainly look for genera which 

 are to be rigidly pronounced. The question therefore 

 naturally suggests itself, — Is this in harmony with what 

 we see; or, in other words, is it consistent with ex- 

 perience, or not? I beheve that it is; for I think it 

 will be found, on inquiry, that the greater proportion of 

 those groups which are more especially isolated in their 

 character (I do not say, necessarily, the most anomalous ; 

 though this in some measure follows from the fact of 

 their detachment) are peculiar to countries which are 



But, however important an element, in the eradica- 

 tion of species, submergence may be; we must not 

 entirely omit to notice other methods also, through the 



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