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 maybe 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- 
