178 



medium of which genera may become well-defined. We 

 should recollect that the removal of a very few links 

 from an endemic cluster is sufficient to cause its dis- 

 junction from the type to which it is next akin, and that 

 where the creatures which unite in composing it are of 

 slow diffasive powers, or sedentary habits, the elimina- 

 tion of such links is (tlirough the smallness of the areas 

 which have been overspread) a comparatively easy opera- 

 tion. The accidental introduction of organic beings 

 amongst others to the interests of which they are hostile, 

 may be a powerful means, as Mr. Darwin has suggested, 

 of keeping the latter in check, and of finally destroying 

 them*. The gradual upheaval of a tract which has 

 been well-stored with specific centres of radiation, 

 created expressly for itself, may (through the climatal 

 changes which have been brought about) succeed in 

 extirpating races innumerable, — those only surviving 

 which are able to adapt themselves to the altered condi- 

 tions j and which would now be consequently looked 

 upon as abrupt topographical assemblages. The over- 



* A familiar example of this disappearance of a creature before 

 the aggressive powers of another, which is either hostile to or stronger 

 than itself, is presented by the Black Rat {Mus rattus) of our own 

 country, — which is said to have been extremely abundant formerly, 

 but which is now replaced by the common brown (or " Hano- 

 verian ") one of Northern Europe. The British species, however, 

 although it has become extremely scarce, is not yet quite extermi- 

 nated : it has been recorded [vide ' Zoologist,' 611) in Essex, and in 

 Devonshire (' Zoologist,' 2344) ; and it still swarms on a small rock 

 oif Lundy Island, in the Bristol Channel. It is reported, moreover, 

 to have been lately re-introduced at Liverpool. 



