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only ones that will remain are those which have become 

 abrupt from causes which are merely accidental. 



Having now, however, examined the question in its 

 broadest phasis, that is to say, on the supposition that 

 Nature is complete in her several links and parts; I 

 shall perhaps be expected to offer a few passing words 

 on what I have already hinted at, — namely, the possi- 

 bility of genera being absolutely well-defined, even on 

 their outer limits, from accident. Briefly, then, it is 

 through the extinction of species that groups may, in 

 some instances, be abruptly expressed: but, as such 

 contiugences are at aU times Kable (whether from 

 natural or artificial caiases) to happen; it would be 

 unfair to build up our generic definition from examples 

 which are the exception, and not the rule, — and, more 

 than mere " exceptions " (as commonly understood by 

 that term), the result of positive disturbances from 

 without. Yet, that genera thus distinctly bounded, at 

 either end, do actually occur, must be self-evident to 

 any one who has attempted to study the distribution of 

 organic beings with reference to the geological changes 

 which have taken place on the earth's surface ; for it is 

 clear that a vast proportion of the creatures which 

 inhabit our globe came into existence at periods anterior 

 to many of those great convulsions which altered finally 

 the positions of sea and land, apportioniug to each the 

 areas which they now embrace : so that, if generic 

 provinces of radiation (no less than specific centres) be 



