50 BIOLOGICAL SOCIETY OF WASHINGTON. 



descent of marine, as well as of land, animals have terminated during 

 past geological periods, both from catastrophal and cosmical causes ; 

 but we may reasonably assume that all the multitudinous forms 

 which people the sea to-day have been derived by direct lineal de- 

 scent from those earliest forms which the sea contained at the dawn 

 of life upon the earth. At least, if this has not been the case, 

 there is nothing in the nature of the proposition that makes it 

 improbable ; that is, there have been no such changes upon the 

 earth since life began as would at any time have necessarily de- 

 stroyed all, or any considerable part, of the marine life previously 

 existing. 



The lines of descent of land animals have, however, been subject 

 to greater vicissitudes ; and the conditions under which they have 

 originated and been perpetuated have been more various than those 

 which have prevailed in the sea. Still, one may readily understand 

 how land animals, which may have occapied a given region of the 

 earth at any geological period when the physical conditions of the 

 land which they occupied were changing, may, by their power of 

 locomotion, have shifted to more congenial places, because, as a rule, 

 such changes have not been too rapid to hasten unduly even the 

 proverbially slow-moving snail. Thus land and palustral air-breath- 

 ing moUusks, although they all require a moist habitat, could easily 

 migrate to other congenial ground, as the land they were occupying 

 may have become too dry for them or may have subsided beneath 

 the sea. Therefore, their migration has always been practically 

 unrestricted; and if, as is believed to have been the case, conti- 

 nental areas have been continuous, though subject to material 

 changes and shiftings from early geological times, there appears to 

 be no reason why, at least, many genetic lines of those animals 

 should not have been continued from tho.se ancient times to the 

 present. 



The case has been quite different as regards true freah-water fishes 

 and fresh-water gill-bearing mollusks, all of which can, of course, 

 exist only in fluviatile and lacustrine waters. When we consider how 



