482 NON-MARINE FOSSIL MOLLUSCA. 



conclude that if every molluscan species that now exists in the sea has 

 not been lineally derived from the earliest molluscan forms that have 

 existed in it, there have been no such changes of its physical conditions 

 as would preclude such a possibility. 



When we come to the study of the fossil pulmouate mollusca, especi- 

 ally the laud-shells, we have also little or no difficulty in understanding 

 how it has been possible for continuous lines of existence of these 

 mollusks to be preserved through successive geological periods upon 

 any continental area, such for example as North America, notwithstand- 

 ing the numerous and great physical changes that have taken place 

 within its area during those periods. Being air-breathers, nothing has 

 apparently occurred to prevent their safe migration to other ground 

 whenever that which they may have at any time occupied became" un- 

 congenial by reason of physical changes, because, as a rule, those 

 changes were effected so slowly that a continuity of congenial habitat 

 for such mollusks was not necessarily broken. They were thus appar- 

 ently as capable of preserving a continuous existence through succes- 

 sive geological periods as the marine mollusca were. 



But, as before intimated, when we come to the study of the fossil 

 shells of the fresh- water gill-bearing mollusca, which in their living 

 state must necessarily have been confined to fluvatile and lacustrine 

 waters, it is not easy to understand, without a special explanation, how 

 continuous genetic lines could have been preserved (as we find they were 

 preserved even down to the present time) through a succession of geolo- 

 gical periods, during which the great lakes, as we know, and all the 

 rivers, as is generally but erroneously believed, in which those mollusks 

 lived, have been successively obliterated. * Elvers are separated from 

 each other by intervening land, and, running to the sea, their mouths 

 are separated by marine waters, neither of which barriers are fresh-water 

 gill-bearing molluska capable of passing. But if it can be shown that 

 throughout those geological periods and down to the present time there 

 has been direct continuity of fresh water by means of lakes or rivers, 

 or both, the case is plain enough. Indeed, as precarious as the exis- 

 tence of continuous life of that kind may seem to have been, under the 

 circumstances of such vast physical changes as are known to have oc- 

 curred, we are forced to conclude that it is in this direction that we 

 must seek for an explanation of the manner in which were preserved 



* It may be suggested that the distribution of these forms from one river or river 

 system to another, may have taken place by the transportation of the mollusks or 

 their eggs by aquatic birds. While such transportation is admitted to have been 

 possible in some cases, it cannot be admitted as a probable cause of any considerable 

 part of the distribution that must have occurred during the several geological epochs 

 in which the molluscan types referred to are known to have existed. Notwithstand- 

 ing the annual migration of myriads of aquatic birds between the northern and 

 southern portions of North America at the present time, and doubtless also ever since 

 It has been a continent, the fresh -water molluscan failure of those regions, respectively, 

 are still distinct. 



