480 NON-MARINE FOSSIL MOLLUSCA. 



article it will be proper to present some general discussion of the nature 

 of the relations which these fossil forms bear to each other and to those 

 now living, and to make some suggestions as to the probable lines of 

 descent of the latter, and the causes of the destruction of those which 

 have failed to survive. 



In the present state of general knowledge of the subject, it seems 

 hardly necessary to state in detail that these relations are regarded by 

 naturalists as genetic in their character, and that the various forms 

 which we discover, both living and fossil, have been evolved from pre- 

 existing primary forms. If the theory of the evolution of organic things 

 is to be accepted in any sense, it seems necessary to conclude that mol- 

 luscan life began in the sea, and that all fresh- water and land mollusca 

 have been primarily derived from those of marine origin.* 



While it is not my purpose to discuss this theory as such, it will be 

 necessary, in the course of the remarks which are to follow, not only to 

 refer to some of the phases which the subject presents when viewed in 

 relation to the probable origin and distribution of the non-marine fossil 

 mollusca, but it will also be necessary to adopt that theory at least as a 

 working and explanatory hypothesis. Indeed, without an hypothesis of 

 this kind, the structural relations which we find to exist between those 

 fossil forms among themselves and between the fossil and living forms 

 are meaningless and unimportant. With the acceptance of such a theory 

 investigations like these become part of the elucidation of a grand his- 

 tory which has fortunately been self registering and subject to errors 

 only of interpretation arising mainly from imperfection of evidence. 



If, as has been premised, the first inolluscan life began in the sea, and 

 the first non-marine mollusca were evolved from those which originated 

 in marine waters, we may well inquire whether at least a part of those 

 non marine forms which subsequently existed had not also a similar 

 origin, and whether we may not consequently regard the great tide of 



* It is well known that in the case of some families of living mollusca, while cer- 

 tain species will thrive in brackish waters, none of them will live in perfectly fresh 

 waters. Living examples of this fact are common, and some have been observed 

 among the Lararnie fauna. But too many facts are known concerning the ability of 

 other mollusks to pass from saline to fresh waters, to leave room for reasonable doubt 

 that the fresh-Water mollusca were primarily derived from the marine. Moreover, the 

 evidence seems conclusive that many species whose living congeners are found exclus- 

 ively in fresh waters were in former geological epochs able to live in waters that were 

 more or less saline. It is the opinion of the writer also that in all or nearly all cases 

 where marine types of mollusks, fishes, &c., have been found in brackish and fresh 

 waters, or their remains in deposits of such origin, they have become habituated to 

 such waters by having been landlocked by the rising sea-bottom around them. That 

 is, they were probably forced to conform to a change of habitat that they would not 

 have voluntarily chosen. It is probable also that, as a rule, the fresh water mollusca 

 have been developed as such by compulsion of this character and not by voluntarily 

 forsaking marine for fresh waters. Once developed as purely fresh water mollusca, 

 certain forms at least became incapable of a return to marine waters; but certain 

 other forms seemed capable of surviving a partial but not complete freshening of the 

 waters. 



