82 FRESH-WATER ANIMALS 



and frogs, form perhaps the most purely fresh- 

 water group of animals known. They are, with 

 very few exceptions, aquatic, at any rate in their 

 earlier or tadpole stages ; and yet no single 

 Amphibian is found in salt water. 



The general conclusion we arrive at is that only 

 a very small proportion of the families of marine 

 animals have made their way into fresh water. Of 

 the eight large groups into which the animal 

 kingdom is divided, one is exclusively marine, two 

 others have an exceedingly small number of fresh- 

 water representatives, whilst in the remainder, 

 fresh-water forms, though more abundant, are con- 

 fined to certain families. 



It becomes now of interest to enquire why it is 

 that the bulk of marine animals do not make their 

 way up the rivers, and thereby escape from the 

 enemies that devour them in such enormous 

 numbers. For a long time it was supposed that 

 the real and sufficient reason was that marine 

 animals are unable to live in fresh water ; but it 

 is now known that though this may apply to some 

 cases, it certainly will not to all. A. series of 

 remarkable experiments were made some years 

 ago, at Marseilles, by Beudant. He took a large 

 number of specimens of different species of marine 

 snails and bivalves, and by gradually adding fresh 

 water to the salt water in which they were origin- 

 ally living he succeeded in gradually converting 

 them into fresh-water animals. During the experi- 

 ments only 37 per cent, of the animals died, 63 

 per cent, surviving the change from sea water to 



