RECENT FRESHWATER INVESTIGATIONS 666 



diatoms in question are those which grow on the shore plants 

 and only rarely the plankton forms. Strodtmann (97) has un- 

 dertaken valuable statistical investigations on fish food accord- 

 ing to which certain species are clearly plankton eaters while 

 others depend upon littoral forms for food. Here also Walter 

 (96b), Greve (97), Droscher (97a). Recently, Brockmeier (98) 

 has observed that in some instances gastropods make direct use 

 of the plankton as food. 



The problem of the origin of the freshwater fauna has been 

 attacked from many sides. Of a general character may be 

 noted the discussion of Forel (94) who holds that the littoral 

 fauna has come by immigration from the marsh and river, 

 the limnetic has been brought in by birds largely and the 

 abyssal has been differentiated in the lake itself. The distri- 

 bution of freshwater animals according to de Guerne (93) is 

 influenced (1) by geographic reasons, (2) by zoologic features 

 of the organism. Even to the weakest a current forms no 

 barrier. 



A number of observers have studied brackish water basins 

 and the modifications in transition from marine to freshwater 

 conditions. Levander (94a) found in an inlet of the sea typical 

 marine and brackish water forms together with those of the 

 freshwater in about equal numbers. Field (97) studied a long 

 pond manifesting all degrees of salinity and found the greatest 

 number of forms near the point of mean salinity. The forms, 

 however, were more like the hali- than the limno-plankton. 

 Ah increase in size of various species on prolonged stay in 

 brackish water indicates one possibility in the production of 

 species. Butschinsky (96) noted the mixed character of the 

 fauna in somewhat similar brackish lakes and its variation with 

 changes in the concentration of the salt. Lemmermann (98) 

 investigated a strand lake which had been dammed and in 

 which the salinity was constantly decreasing. The plankton 

 which was notably rich manifested some irregularities traceable 

 to the variable salt content of the water and certain species 

 were localized at various places. 



