RELICT FAUNA OF GREAT LAKES 
49 
crustacean, called Pontoporeia lioyi, 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 latter 
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 marine 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 
taken, besides that of the Great Lakes, is in tidal pools on the 
west coast of Hudson Bay. 
* Peschel, O., “ Vergleichende Erdkunde,” p. 167. 
t Credner, R., “ Reliktenseen,” I., p. 105. 
L.A. 
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