84 FRESH-WATER ANIMALS 



periments have been pushed still further : and by 

 addition of fresh water Artemia salina has been 

 converted into the fresh-water genus Branchipus, 

 which is of larger size, and in many respects very 

 different. 



A number of other cases could readily be quoted 

 illustrating the same point, namely, that many 

 marine animals which never do make their way 

 into rivers, are quite capable of living in fresh 

 water, if the change is made sufficiently gradually. 

 If therefore it is not the difference between sea 

 water and fresh water that prevents marine animals 

 from entering rivers, there must be some other 

 cause or causes at work. One of these is, un- 

 doubtedly, the severity of the climatic conditions 

 to which fresh- water animals are exposed, as 

 compared with the practically uniform temperature 

 of the sea. Further, the variation in the amount 

 of water in rivers at times of drought and flood 

 respectively, the changes in the strength of the 

 current and modifications in the character of the 

 water from sewage and other contamination, all 

 conspire to render the environment a very shifting 

 one and at times a very harmful one, as compared 

 with the much greater uniformity of external con- 

 ditions under which the marine fauna exists. 



But even these causes, potent as they un- 

 doubtedly are, will not suffice to account for so 

 few marine families getting into fresh water. 

 Some further explanation is required, and this is, 

 I believe, to be found in a suggestion made 

 originally by Professor Sollas, of Dublin, who has 



