THEIR POSITION IN THE ANIMAL KINGDOM. 11 



lakes and rivers. Of a large number of inland 

 lakes we know that it is only within a period 

 scarcely separable from the present that they have 

 become detached from the sea. A large portion of 

 the inhabitants of these lakes is accounted for by 

 this very fact. In regard to the rivers we have 

 here specially to consider fishes and mussels. Now 

 it is well known that a number of fishes for 

 instance, the salmon, smelt, eel, and certain kinds 

 of plaice spend their life partly in fresh and partly 

 in salt water, according to the season of the year, 

 for the sake of propagation ; and further, that they 

 can be transferred from the one to the other kind 

 of water without injury. Hence we may assume, 

 in the case of all purely fresh-water fishes and 

 mussels, that their progenitors could also at one 

 time live in either kind of water : we have thus a 

 perfectly satisfactory explanation of the occurrence 

 of the same genera, partly also of the same species, 

 in rivers situated very far apart. Examples of 

 this kind in the group of mammals are not fre- 

 quent, but instructive. The sea-cow, discovered 

 by Vogel in the Benue, is the only species of the 

 order of the Sirenia which, it would seem, has 

 never attained a fuller development, and to use 

 an expression of Kiitimeyer's has completely taken 



