Salt Marshes and their Inhabitants. . 33 



they will sometimes come andbask almost in tlie palm of the hand; 

 probably the warmth of the hand is the attracting influence. 



The species of Mysis, or "Opposum shrimp/' mentioned 

 above, living as it does both in fresh and strongly brackish 

 water, brings before ns a very interesting problem, and one by 

 no means easy of accurate solution, yet concerning which we 

 have some few data which may guide us to a right result. We 

 find that most fresh- water genera possess also some represen- 

 tatives inhabiting the sea. And it at once strikes us that it 

 must be something more than a merely fortuitous coincidence 

 by which animals so far separated in their habits agree so 

 closely in structure as to be included in the same genus. If 

 Mr. Darwin is right, as we believe he is, in supposing that at 

 least all genera of the same order aredescendedfrom one common 

 ancestor, we must seek for an explanation of the present state 

 of things by looking backward to some remote period when the 

 progenitors of the existent fresh water and marine forms were 

 not separated by the impassable barriers which now divide 

 them. We extract the following interesting remarks on this 

 subject from Messrs. Spence Bate, and Westwood's History of 

 the British Sessile-eyed Crustacea (vol. i. p. 390). With refer- 

 ence to the facts which we have mentioned, these authors say : 

 " The key may be suggested by the interesting discoveries of 

 Cedarstrom, Olofson, and Widigrew, in the lakes of Yetter and 

 Yener, in the south of Sweden, of which an account has been 

 ■published by Loven. These two inland fresh- water lakes are 

 ■situated on high ground, and have the surface of their waters 

 300 feet above the level of the Baltic, whereas the bottom is 

 120 feet below such level. In these lakes (which appear to 

 have been lifted up with the gradual uprising of the country) 

 have been found several genera and species of Crustacea, three 

 of which are Amphipoda, which are affirmed to be identical 

 with marine ones, namely, Oammaracanthus loricatus (Sabine, 

 Ross, Kroyer), Pontoporeia affinis (Lindstrom), and Gammarus 

 cancelloides (Grerstfeldt) . The first is now only known to exist 

 in the Arctic seas, the second in the Baltic, and the last was 

 found in Lake Baikal, in Central Asia. It is therefore sug- 

 gested by Loven that when the land was raised so as to convert 

 these waters from marine bays into inland lakes, these marine 

 species were retained within the basins, the waters of which 

 have since been changed, through the agency of springs, into 

 fresh- water ; and with the gradual transfer of the water the 

 habits of the animals have also changed gradually, and that 

 without any outward alteration of form. Professor Loven thinks 

 that there is sufficient evidence to show that this change in the 

 conditions of these lakes must have taken place during the 

 great glacial period, at a time when the animals now found in 

 vol. v. — NO. I. D 



