I 



Ch. XLI.l EEFEEENCE TO THE OEIGIN OF SPECIES. 



425 



of this shell 



among the fossils 



im 



Madeira 



These three distinct though kindred forms of a " peculiar 

 division of the HelicidsG belonging to Madeira, Porto Santo 

 and the Dezertas remind us of the representative species 

 of some genera found in Asia, Europe, and America. 



Having alluded to the Dezertas, I may add that 19 species 

 of landshells have been found on them, 1 2 of which, or about 

 two-thirds of the whole, are common to Madeira, and only 



6 to Porto Santo. The nearer affinity 

 Madeira was to be expected, not only beca 



the fauna to 



proximity, but because, as will be seen by our map, Madeira 

 and the Dezertas stand within the same 100 fathom line, 

 and the channel between them may once have been narrower, 

 although there is no reason for believing that the land was 

 ever continuous, or even that Chao, Dezerta Grande, and 



united 



some 



It 



is worth remarking, as showing the limited range 



of 



species when the whole archipelago is considered, that there 



common 



Madeira 



Madeira 



unmistakable 



newest lava streams ; for to say nothing of the time required 

 to annihilate several species and greatly to alter the relative 

 numbers of others, there are proofs of local geographical 



of subsequent date. Since the accumulation of 



changes 



the volcanic sand 



mud, in which the landshells are 

 1 much undermininor of the sea-cliffs. 



promontory 



and on the northern coast of Porto Santo. Some of the 



shelly formation of the 



mentioned island consists of 



sand-dunes which have been cut off abruptly in the vertical 



must 



direction. 



suffered great denudation 



farther in a seaward 



The whole island, indeed, of Porto Santo has 



some 



letters 



map (fig. 137), one of them 



now covered by only 26 feet of water, and the other the 



