Ch. XLI.] 



EEFERENCE TO THE OEIGIN OF SPECIES. 



427 



T 



^ 



■a 



^i 





I 



marine channels liave formed in most cases impassable barriers. 

 The Scilly islands are as far from Cornwall as is Madeira 

 from Porto Santo^ yet in them the conchologist obtains no 



distinct species, nor even 



from Madeira to 



marl 



whereas, on 

 Santo, he finds fonr-fifths 



of the species different, besides some peculiar races, even of 

 those shells which are common to the two sides of the 



channel. 



mav 



IT. cartitsianay H 



England display a richer fanna, and contain certain species 



(about eight), which do not range farther northwards than 



Yorkshire. These are; Helix pomatia, 



revelata, H. Pisana, H. ohvoluta, Bulimiis 



BodolpU, and G. Uplicata. It is more 



species which are peculiar to the north. Vertigo alpestris 



difficult to name 



exam 



manner 



same 



law of distribution the apparently incongruous 

 phenomena exhibited in the two regions above compared ? 

 Some zoologists who have been struck with the unusual 



number 



endemic 



marked 



mol 



lusca must be more variable than other classes of the 



animal kinerdom 



inadmissible 



s of Madeira 



remarkable 



m of the grenera Helix 



times. 



from the Newer ] 

 e enigma we must 

 LT)se of time, durir 



the British and those of the Atlantic archipelagos have 

 remained separate from each other and from the nearest con- 

 tinents. In the one case we have to deal with thousands, in 



the 



other with millions, of years. f In the one case there 

 has been everywhere a land communication between every 

 part of the archipelago since the commencement of the 



marine 



and terrestrial 

 testacea were everywhere the same as they are now ; in the 



* See Mr. J. Gwyr 



Conchology, 1866-67. 



t See above, Vol. I. p. 300. 



