EELICT FAUNA OF GEE AT LAKES 49 



crustacean, called Pontoporeia hoyi, allied to a marine form, 

 inhabits the Lakes Michigan and Superior, while in the 

 former still another marine form (Pontoporeia filicornis) and a 

 remarkable fish called Triglopsis thompsoni have been met with. 

 Professor Peschel first applied the term " relict lakes " * 

 to lakes like those referred to, because they contain the 

 relicts of a former sea, and the word has since been largely 

 adopted in the sense in which it was first proposed. Never- 

 theless, some authorities, notably Professor Credner,f who 

 treated the subject most exhaustively, maintain that these 

 so-called relict animals have either immigrated to the fresh- 

 water lakes directly from the sea, or have been transported! 

 from their marine habitat by accidental means. He notes 

 with surprise the absence from all the so-called relict lakes 

 of marine mollusks or species allied to such, contending that 

 some of them ought to be found in these lakes, if the latten 

 had ever been covered by the sea. Professor Credner's argu- 

 ments against the relict nature of such lakes as Superior and 

 Michigan, seem at first sight very convincing. But zoo- 

 logists are now well acquainted with' the fact that all marine 

 mollusks have free-swimming larvae. These being altogether 

 a prey to currents would infallibly be swept into the sea if 

 placed into running fresh water. Thus the parent marine 

 mollusks would be unable to propagate their kind in fresh 

 water even if they could adapt themselves to it. This circum- 

 stance seems to account in a satisfactory manner for the 

 absence of marine mollusks from relict lakes. Quite apart 

 from this objection to the relict theory, Professor Credner's 

 contention of an active migration of m'arine organisms to 

 fresh water is particularly inapplicable in the case we have 

 been considering, because none of the ''relict species" 

 alluded to occur in the lower lake of Ontario, which, moreover, 

 is separated from the upper lakes by the impassable Niagara 

 waterfall. It is of interest also to note that the only other 

 locality in which the fish Triglopsis thompsoni has been 



bken, besides that of the Great Lakes, is in tidal pools on the 



r est coast of Hudson Bay. 



* Peschel, O., " Vergleichende Erdkunde," p. 167. 

 t Credner, E., " Beliktenseen," I., p. 105. 



L.A. E 



