154 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



duals of various species which were gradually accustomed to 

 fresh water, only 37 per cent, died, while of a corresponding 

 number which were kept at the same time constantly in sea- 

 water 34 per cent. died. Thus the percentage of mortality in 

 the group of animals that were gradually accustomed to a 

 foreign element was only three per cent, higher than in those 

 which remained in their natural element. Certainly it must be 

 considered that this result was due to the circumstance that 

 certain species as Myiilus remained altogether unaffected, 

 while others died out entirely. For further details I refer the 

 reader to the note. 60 



We perceive from these experiments of Beudant's that some 

 species of Molluscs can live equally well in fresh and salt water, 

 although they may be exclusively fresh-water or marine forms. 

 Unfortunately the experiments have not been carried out far 

 enough for us to be able to draw any far-reaching conclxisions 

 from them. Beudant, it is true, proved that a fully grown 

 Mytilus could be accustomed to fresh water, but not that it 

 could multiply in it. Granting that a gradual transformation 

 of the salt water in the Baltic into fresh water could take place, 

 according to Beudant's experiments a number of full-grown 

 or half-grown animals might become accustomed to the fresh 

 water ; but the species might nevertheless very possibly die out, 

 particularly if their eggs and larvae were not equally capable of 

 surviving in fresh water. In the quaternary period numerous 

 oyster-beds existed in the Baltic which have since then entir ely 

 disappeared ; 6l and yet the oyster belongs, according to Beudar it's 

 tables, to those forms which are able to live almost as well in 

 pure fresh water as in salt water. The extinction of the oyster 

 in the Baltic may have resulted, as must certainly be admitted, 

 from a variety of causes ; but in view of the total absence oi* 

 all means of proof we must not reject as unfounded the 

 assumption that it was caused by the incapacity of the young 

 oyster-larvae to withstand the injurious effects of the diminution 

 of salt in the Baltic. 



Plateau went somewhat further than Beudant in his re- 

 searches on the aquatic Articulata. His experiments on the 

 common Water-Louse (A sellus aquaticus) are particularly Interest- 



