white.] GENERAL DISCUSSION. 75 



non-marine molluscan life that has come down to us from the past as 

 having been made up of many contributing streams which had the sea as 

 their source, rather than regard it, figuratively speaking, as a stream 

 having an ancient and perhaps single source, augmenting in volume 

 and throwing off from itself numerous branches which became collateral 

 lines of descent. It is reasonable to suppose that both these conditions 

 of evolution have prevailed, namely, that while some of the genetic Hues 

 of the non-marine mollusca may, and doubtless do, run back to paleozoic 

 time, the families of non-marine mollusks have had many accessions 

 from the sea from time to time during the successive geological ages, 

 and that each of these accessions was, at least potentially, the source of 

 subsequent collateral lines of descent. But the question of the primary 

 origin of these lines of descent is not an essential one in these discus- 

 sions, because I propose now only to consider the evidence that they 

 have existed, and make some inquiry into the character and relations of 

 certain of those lines which may be regarded as directly lineal. In 

 these inquiries I shall treat coincidences of structure among the fossil 

 and recent shells which come under consideration not only as indica- 

 tions but as proof of genetic relationship ; and this relationship as funda- 

 mentally pertaining to the geological and zoological history of the mol- 

 luscan life which they represent. 



It is true that the incompleteness of the material which is or that we 

 may ever hope to have available for study is so great as to leave many 

 gaps, at least in the details, of the life-histories of all the types which 

 have been recognized among the fossil mollusca. 



When, therefore, we come to trace the probable lines of succession 

 of the various types of non-marine mollusks, we are met with certain 

 difficulties, both apparent and real ; and in endeavoring to account for 

 the manner in which those lines may have been preserved unbroken 

 through successive geological periods, the difficulty seems especially 

 great, when casually considered, in the case of the fresh-water gill- 

 bearing mollusca. 



The prevalence of the sea has always been practically universal ; and 

 the various movements which the earth's crust has undergone since life 

 began in the sea, while they have repeatedly disturbed or destroyed the 

 habitats of its molluscan denizens in certain localities, and have broken 

 many of the lines of genetic succession of types that had from time to 

 time become established, there has evidently never been anything like 

 such a general destruction of life in the sea as would either break or 

 materially interfere with the greater part of the principal lines of such 

 succession. In short, the marine field for the development and perpe 

 tuity of molluscan life has been ample and unbroken from the begiuning 

 to the present time, and we are at no loss to understand how continuous 

 lines of genetic succession of its denizens may have extended down 

 through all the geological ages, modified, it is true, by immediately 

 environing and cosinical causes, but still unbroken. We may at least 



