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 diffusive powers, or sedentary habits, the elimina- 

 tion of such links is (through 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 ; 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 

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

 to have been lately re-introduced at Liverpool. 



