9 6 FRESH-WATER ANIMALS 



descended from marine animals which have become 

 habituated to fresh-water life: that very little 

 modification, if any, is required in the adult 

 structure to fit an animal for the change in habitat; 

 and that many purely marine animals will live readily 

 in fresh-water, if the change from one to another is 

 sufficiently gradual. We have further seen that the 

 reasons why so few groups of marine animals have 

 given rise to fresh-water representatives, concern 

 not so much the adult condition as the earlier larval 

 stages ; and that it is the inability of small free- 

 swimming ciliated larvae to hold their own against 

 the currents in the streams and rivers that is pro- 

 bably the main cause of this paucity of fresh-water 

 species. 



An examination of the life-history of those forms 

 that have established themselves in fresh-water, 

 has shown that in many cases there are very 

 special devices of a curiously interesting kind to 

 enable the animals to get over these difficulties. 

 We have only had time to notice a few of the 

 more striking instances, and very much yet remains 

 to be done before we can explain fully all the 

 peculiarities of the fresh-water fauna. 



The aspect of the question which I have 

 touched on this evening, has only very recently 

 attracted notice ; the subject is one of great im- 

 portance and interest, and I would venture to 

 commend it very earnestly to the attention of 

 members of the Society, as one the further and 

 more systematic investigation of which would, 

 beyond doubt, yield results of high scientific value, 



