106 Sotitas— 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 (Cenomanian or Senonian in age) have been 
obtained—Unio, Physa (one sp.), Paludina (twelve sp.), Valvata (four sp.), Limnza 
(six sp.), and Pisidium (one sp.) 
Thus we find several freshwater genera of Mollusca already distributed in 
Cretaceous times over parts of the Palearctic, 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 Littorinide, 
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 (7. e. subsequent 
in time to the upheaval of the Siwalik hills, and previous to the upheaval of the 
Sub-Apennines). The Mytilidz 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 as 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, 
