430 



INSULAR FLOEAS AND FAUNAS WITH 



[Cii. XLL 



same 



to cross with, the first settlers and check divergence. 



timber 



any other 



means 



their agency must be so casual and irregular as to cause the 

 results to appear capricious in the extreme. 



The first Miocene Helix which reached Madeira may have 

 been of a different species from the first which reached Porto 



Santo. 



I ^ 



o 



Helix inflexa Martens 



m 



H. vortosanctana. of which the OT^antic H. 



Madeira 



mentioned form 



r 



Ktinct H. Baymo 



er Miocene stra 



fossil and recent in 



to have been the ancestral type of another common shelly 

 H. Bowditchiana Pfeiffer^ found both. 

 Madeira and Porto Santo. 



Let us assume that certain Miocene species^ nearly all 

 of them long since extinct^ were carried as waifs and strays 

 to separate islands by a concurrence of circumstances so 

 rare as to happen once only in several hundred thousand 

 years^ other combinations of circumstances almost equally 

 rare might be required to convey a species from one island 

 to another. A volcanic eruption^ for example^ which might 

 only occur once in the whole course of the building up of 

 an archipelago, at exactly the same season of the year, or 

 at the same height above the sea, with equal violence and 



same 



Such a convulsion mi 



cause the dispersion of some 



Helices from one part of an archipelago to another in a 

 manner altogether without parallel during the antecedent 



res^ion. If the reader will 



same 



Monte Nuovo 



p. 608, near Naples, in 1538, he will see that while many 

 land-birds were killed, those which escaped and flew terrified 



om 



the scene of the catastrophe, must, like the human 

 inhabitants, have been covered with mud which was showered 

 down so as to envelope all things. In the beginning of such 

 an eruption trees, shrubs, and vegetable soil, in which the 



