of North Africa with South Europe. 433 



remnants of which still continue to exist on our high mountains, 

 like an elevated stratum overlying all the others which live be- 

 neath. These lowest are, on the one hand, the western fauna, 

 which we designate the Lusitanian, the types of which are the 

 forms common to the North of Africa and Europe; on the other, 

 the eastern fauna, which we may perhaps venture to call the 

 Asiatic, and which is broken up into several members, depend- 

 ing on the physical differences existing, for example, between 

 the Caspian Steppes and Asia Minor. 



It is not my intention here to point out what relations the 

 superposition of the individual faunas of the European seas bear 

 to this ; but we may draw attention to the fact that the Mollusca 

 which Vienna has in common with Senegambia (as Twjonia 

 anatina) were without doubt formerly inhabitants of some parts 

 of the present Mediterranean east of Sicily, probably became 

 extinct during the diluvial epoch, and were subsequently unable 

 to regain their former abode. M'Andrew, it is true, informs 

 us that, favoured by the current, some tropical species, as, 

 for instance, Cymha alia, make their way through the Straits 

 of Gibraltar to the North-African coast ; but they do not pro- 

 ceed very far, and the character of the Mediterranean fauna 

 is totally distinct from the Senegambian. We are accustomed 

 to consider climatic variations as the essential cause of all these 

 displacements of land- and sea-faunas and floras; and some 

 distinguished naturalists in Switzerland, impressed with the 

 great effects which the sirocco produces on their glaciers, have 

 thought that they could explain the greater extension of the ice- 

 masses in former times by its absence. In this way they have 

 arrived at the same result as that obtained by the study of 

 palaeontology, geology, and geographical distribution — namely, 

 that the Sahara Desert, the source of the sirocco, was once co- 

 vered with water. Upon the heights of a continental Europe a 

 more severe climate may certainly have been produced by such 

 a cause ; but for Europe, broken up into a sort of archipelago 

 (such as we must imagine it to have been at the time when the 

 Senegambian Mollusca of to-day were living near Vienna), no 

 great lowering of the temperature of the sea could have resulted, 

 and the whole archipelago had, without doubt, notwithstanding 

 the absence of the sirocco, a moderate sea-climate. 



Questions and doubts still arise on all sides, but we are at 

 least able to form some idea of the path we have to pursue in 

 studying the origin of the present creation from the preceding, 

 and by which it will be possible to arrive at a more correct con- 

 ception of the repeated changes of the organic world. 



