242 Dr. Max Weber on the 



not in itself a proof of greater antiquity. In the case of more 

 lowly organized animals the formation of species is no more 

 limited in time than in that of m ore highly organized forms. 



Now in my opinion it is possible to demonstate the 

 marine origin of a considerable fraction of the freshwater 

 animals which are of regional occurrence. Naturally I am 

 not speaking here of the hypothesis that in the end all fresli- 

 water animals are derived from the primeval ocean. 



In the first instance I am thinking of the " relic-animals " 

 (Relicten-Thiere), yet only in the sense in which they are so 

 regarded by Lovdn and Credner. Accordingly I consider as 

 " relics " only such animals as are really of a marine nature 

 and inhabit freshwater basins, that can be proved by geology 

 to have been once filled with sea-water and to have been in 

 connexion with the ocean. The numerous " relic-lakes," 

 with which zoological fancy has covered the earth, for the 

 most part do not bear geological criticism, any more indeed 

 than they sustain more precise zoological examination. 

 Besides a number, small it is true, of genuine marine relics, 

 fresh water is peopled by numerous marine forms ; these, 

 however, have not been '' left behind," but are immigrants, 

 active or jjassive, as the case may be. 



I would therefore divide the freshwater fauna as follows : — 



1. Universal freshwater animals. 



2. Regional „ „ 



a. Local genuine freshwater animals, which form 



an already ancient stock. 



b. Marine forms. 

 a. Relics. 



)8. Immigrants. 



^1. Active immigrants. 

 /Sg. Passive „ 



The larger portion of these immigrants wander actively 

 from the sea into the brackish estuaries and further into the 

 lower course of the rivers, to gradually ascend until they 

 penetrate far into the interior. From the nature of the case 

 the role played by passive immigrants is a subordinate one, 

 since they are carried almost exclusively as parasites from the 

 sea into the rivers, and in this manner finally also reach the 

 lakes, like Tachaa lacustris and Rocinela typus, which will be 

 dealt with later on. A certain participation on the part of 

 the animal that is passively transported is self-evident. 



The communications of Giinther, Sauvage, von Kennel, 

 and Stuhlmann already point to the fact that the marine 

 immigrants in the tropics at any rate play an important 

 part, as is even still the case ; for I am convinced that 



