424 



INSULAE FLOEAS AND FAUNAS WITH 



[Cii. XLI 



, 111 the sliellj sand of Porto Santo a conspicuous sliell, 

 Helix Lowei, is very abundant. It is of so large a size that 

 it could hardly have escaped detection if it still existed on 

 either of the principal islands, but lately a few individuals 

 of this species have been detected on the rock called Ilheo 

 di Cima off Porto Santo.^ By some concholo gists Helix 

 Lowei is regarded as a gigantic variety of tlie living H. 



occurs fossil in the same sands. 



means 



Porto sanctanay whicli also 

 If this opinion be correct, it offers 



example in the fauna of this archipelago of the same distinct 

 aces being found both fossil and recent, and in both cases 

 without any intermediate varieties. One of the two forms 

 may possibly represent the parent stock, and the other the 

 extreme of divergence. There must once have existed, 



I 



Natural 



tional forms between the two 



emes. But these forms 



may have died out for want of favourable conditions, or may 



extremes 



m 



before alluded to (p. 323), according to which more plants or 

 animals find support in a limited area if they are of many 

 different genera then if they all belong to one genus. There 

 are however in the Madeiran archinelap^o some nolvmornhous 



Helix 



missm 



us of the varieties of the English brambles and roses ; but 

 such cases are the exception to the rule, for reasons to be 

 explained in the next chapter. 



I have alluded to Helix tiarella in Madeira ; an allied re- 

 j)resentative of the same peculiar form, H. coronata^ abounds 

 in a fossil state in Porto Santo, and is also still living in 

 that island, though it is rare. Another, or third closely allied 

 species, H. coronula^ was first found fossil in Bugio, one of 

 the Dezertas, and it probably still exists on some part of those 

 inaccessible rocks, for a few living individuals have lately been 



found on the nearest adjoining coas 

 may supply an example of the smaller 

 one of its indigenous species to 



Made 



They 



Madeira 



^ See Map, fig. 137, p. 405. 



