26 FRIENDS WORTH KNOWING. 



nearer the north pole than any other known shell. Yet it 

 is a remarkable fact, that, however erratic and extensive 

 may be the range of -the genera to which they belong, the 

 majority of the species of pulrnonates of all sorts have an 

 extremely limited habitat, in some cases comprising only a 

 few square rods. A second noteworthy fact, obtaining in 

 no other extensive group of animals, is, that many more 

 species of land shells exist in the islands than on the conti- 

 nents of the world. Mr. A. R. Wallace accounts for this 

 curious fact by explaining how certain influences make isl- 

 ands particularly if long insulated 



HELICES IN HTJMBLB CIRCUMSTANCES. 



