106 SOLLAS On the Origin of Freshwater Faunas. 



have they changed with the lapse of time. So slightly divergent are these lines, 

 considered as lines of differentiation, that if we bound them all by two imaginary 

 straight lines we should have an evolutional parallax that would carry back the 

 origin of these types to a period inconceivably remote." * 



It is not only in North America that we find, however, the same freshwater 

 genera as in Europe : India also furnishes us with an instructive list : thus from the 

 lower Intertrappean freshwater beds (Cenomaniafi or Senonian in age) have been 

 obtained Unio, Physa (one sp.), Pal udin a (twelve sp.), Valvata (four sp.), Limnaea 

 (six sp.), and Pisidium (one sp.) 



Thus we find several freshwater genera of Mollusca already distributed in 

 Cretaceous times over parts of the Palsearctic, Nearctic, and Oriental regions. 



Proceeding in our review to the Tertiary period, we encounter an epoch of 

 gigantic mountain building, and consequently of extensive lake formation. Many 

 great mountain ranges now existing took their rise after the Eocene, Miocene, and 

 Pliocene periods ; and with them, I doubt not, several of our existing great lakes 

 and inland seas, such as the Caspian, lake Baikal, and the lake system of Central 

 Africa. 



Some marine forms were probably enclosed in these basins, and became con- 

 verted into freshwater genera ; but the majority of the freshwater inhabitants of 

 the Tertiary lakes and rivers were derived from previously-existing freshwater 

 species, as is shown by the fact that they belong to genera already in existence in 

 Mesozoic times. There are, however, some fresh acquisitions : thus the Littorinidae, 

 which might, from their hardy habits and universal distribution, have been expected 

 to have furnished earlier some freshwater species, are now represented by the fresh- 

 water genus Lithoglyphus, which is found in Lower Pliocene strata (i. e. subsequent 

 in time to the upheaval of the Siwalik hills, and previous to the upheaval of the 

 Sub- Apennines). The Mytilidae again, of which the absence of earlier freshwater 

 modifications is equally remarkable, are now represented by numerous species of 

 Dreissena, which first appears in the Upper Eocene. 



Passing now to the existing lakes, which we regard as the modified descendants 

 of Tertiary seas, we find that those of the northern hemisphere have been subjected 

 to singularly unfavourable conditions : thus the lakes of North America, Europe, 

 and Asia have endured all the rigours of a glacial climate, and in some cases have 

 been submerged beneath a glacial sea ; while the Caspian, in addition, suffers from 

 an excessive concentration of its waters. Singularly unwholesome as it has thus 

 become, it yet retains, however, a fragmentary relic of a Tertiary fauna. So much 

 has been written on the subject of the Caspian and its fauna, that I make no excuse 



* Let us add, it by no means follows that we are bound to carry back their origin to a period so 

 determined. The probabilities are that, if we could trace the lines of descent backwards, we should 

 finally find them rapidly converging us they entered a region of geographical change such, for instance, 

 as that of the conversion of a continental sea into a system of freshwater lakes. 



