170 EEV. B. T. Lowe's list of shells 



similar in their flat and sandy cliaracter to those of the opposite 

 continent at Mogador. In other words, there is no present differ- 

 ence in physical character between the shores of Mogador and 

 those of Porto Santo, Grand Canary, Lanzarote, and Fuerteven- 

 tura, corresponding with the marked discrepancy existing between 

 their more ordinary Molluscous inhabitants. Had such difference 

 prevailed now, it might have been supposed sufficient to account 

 for the difference in the shells, as the result of former changes, 

 by dislocation or disruption in the coasts themselves, causing 

 the disappearance from those islands of the Mogadorian-British 

 species. 



Bearing in mind, therefore, that Mogador is fully 18-1- degrees 

 of latitude south of the nearest southern shores of England, and, 

 on the other hand, only about 400 English miles distant in a nearly 

 easterly direction from Madeira or Porto Santo, and not above 300 

 English miles in a north-easterly course from the precisely similar, 

 flat, sandy shores of Lanzarote and Euerteventura, whilst it is 

 removed only one, two, or three degrees of latitude from any one 

 of these*, it would seem that climatal differences exercise far less 

 influence than others of topographical or geographical configm*a- 

 tion on the diffusion of marine Mollusks, if the theory of their 

 progressive propagation be exclusively maintained. For, on that 

 hypothesis, it is evident that these common littoral MoUusks have 

 spread in this instance abundantly and for a vast extent from 

 north to south, in spite of difference of latitude, all down the west 

 coasts of Prance, Spain, and Portugal to Mogador, along the shores 

 of two continuous, or nearly continuous, continents ; whilst they 

 have not found their way westwardly across a comparatively 

 narrow oceanic space or deep-sea channel, from one of these to 

 places situate in very nearly the same parallel of latitude. 



This continuation of North Eiiropean littoral species so far south 

 along the shores of Africa has, however, doubtless been assisted 

 by the prevalent strong northerly winds, sweeping down the coast 

 far below Mogador, and cooling down the surface-temperature of 

 the sea below its average shore-heat in the almost parallel latitudes 

 of the Canary Islands and Madeira. I regret not having leisure 



* Porto Santo, lat. 33° 3' 30" N., long. 16° 20' 14" W. ; Madeira (Funchal), 

 lat. 32° 38' 22" N., long. 16° 54' 56" W., Vidal ; Mogador, lat. 31° 30' N., 

 long. 9° 46' W. ; Lanzarote, lat. 29° 0' N., long. 13° 54' W. ; Fuerteventiira, 

 lat. 28° 3' N., long. 14° 31' W., Johnston. Lanzarote is from 70 to 75, and 

 Fuerteventura only about 50 English miles distant from the yiearest opposite 

 part of Africa. 



