170 FRESHWATER FAUNA 



number of species, over 150 marine, and less than half a 

 dozen fresh water. So it is with the Polyzoa ; some 

 families are unrepresented, of marine species there are 

 nearly 250, of fresh water about a score. The same story 

 is told by the Mollusca ; the cuttlefish are conspicuous 

 absentees, of the bivalves most of the families are missing, 

 the total number of species of molluscs inhabiting our 

 rivers, lakes, and ponds cannot exceed thirty, while in 

 the British seas there are between five and six hundred. 

 Apart from the minute Cladocera, which are more abun- 

 dant in fresh water than salt, the Crustacea exhibit the 

 same contrast. The rule is general, with only one or 

 two exceptions, the Cladocera just mentioned, and the 

 tiny Rotifers, which are predominantly fresh water ; of 

 400 species or more, only seventy are found in the sea. 



The difference which we observe in the case of our own 

 islands is repeated to a greater or less degree all over the 

 world, and, w T hat is perhaps still more extraordinary, a 

 great number, perhaps the majority, of our freshwater 

 genera are the same, or nearly the same, as those which 

 inhabit the rivers and lakes of the greater part of the 

 world. The sameness of the freshwater fauna, almost 

 wherever it is found, increases its contrast with that of 

 the sea, and renders all the more difficult the problem 

 which we must now attempt to solve. 



The problem is evidently manifold. In the first place, 

 we have to explain how it is that marine animals are so 

 largely excluded from our rivers ; in what manner those 

 forms now found in fresh water have obtained their foot- 

 ing, by what means they have attained so wide a dis- 

 tribution, and what cause has prevented them, while 

 exposed to such differences of climate and other conditions, 

 from displaying the same originality as their marine 

 relatives in developing new types of structure. It may 

 not be possible to find answers to all these questions ; we 



