these relict lakes, seems at first sight a stumbling-block to the
theory. But the explanation is really simple enough. It is to Dr. Sollas
that we owe a very ingenious explanation of the origin of freshwater
faunas. He showed that all freshwater organisms in their early stages of
development are provided either with some process enabling them to
attach themselves to a foreign object, or that they pass this period
within the body of the parent. This is a provision of nature to prevent
freshwater organisms from being floated out to sea, where they would
perish, until they reach maturity and can cope with floods and currents.
Had Professor Credner been aware of Dr. Sollas's views, no doubt he
would have modified his criticisms, for, as most marine mollusca have
free-swimming larvae, they would have little chance of becoming permanent
residents of lakes. During their larval stage, marine molluscs are quite
a prey to the currents of the sea. They have practically no swimming
organs, and only move by lashing to and fro the tender cilia with which
they are provided.

[Illustration: Fig. 14--The Four-horned Sting-fish (_Cottus
quadricornis_), reduced from Professor Smitt's figure in the _Fishes of
Scandinavia_.]

This disposes, therefore, of Professor Credner's main criticisms. As for
the fauna of the relict lakes, we are now only concerned with those of
Northern Russia, Finland, and Sweden. In the lakes Wetter and Wener in
the latter country occurs the four-horned sting-fish (_Cottus
quadricornis_, Fig. 14), which, as we have learned, also inhabits the
northern part of the Baltic, and, as was suggested, migrated there at a
time when the latter was connected with the White Sea. The principal
food of this little fish consists in a marine Crustacean called _Idotea
entomon_, an animal allied to our common woodlouse. This is a typical
marine species, but it occurs also in the relict lakes of the countries
mentioned above, as well as in the Baltic and the Caspian. Perhaps the
best known form with a similar range is the Schizopod crustacean _Mysis
relicta_[3] (Fig. 15), which is clearly a descendant of the Arctic
marine _Mysis oculata_, of which it was formerly considered a mere
variety. The two Amphipods _Gammaracanthus relictus_ and _Pontoporeia
affinis_ and the Copepod _Limnocalanus macrurus_, are three additional
well-known Arctic crustaceans whose range differs but little from those
above-mentioned.[4]

[Illustration: Fig. 15