CHAPTER II. 



ON THE NATURE AND ORIGIN OF FRESH-WATER FAUNAS 

 AND THEIR RELATION TO THE FAUNA OF THE SEA. 



It is a fact that there are forms of animals inhabiting 

 fresh-water which do not inhabit the sea, and which die if 

 placed in the salt-water environment of the ocean ; while, 

 conversely, there are animals which habitually live in the sea, 

 and die if they are subjected to the action of water which is 

 fresh. This is a matter of common knowledge, and when 

 we speak of fresh-water and marine faunas we mean to 

 describe in general the animals which can live only in one or 

 other of these media. But it is also a fact that there are 

 animals which can live as well in the sea as in fresh water, 

 such as salmon ; while, again, there are others which can live 

 in the brackish mixtures of salt and fresh water occurring at 

 the mouths of rivers. From a variety of reasons, some 

 palaeontological, some based on the results of comparative 

 anatomy, and which, although they may be rather indefinite, 

 are probably weighty, it is generally conceived by naturalists 

 that all the animals now inhabiting the fresh-waters of the 

 earth originally arose in the sea. Further, it has often been 

 held that these same fresh-water types have arisen during 

 the past through the successful efforts of different marine 

 organisms to colonise fresh-waters from the ocean. And, 

 following the same kind of reasoning, it has often been sug- 



