RECENT FRESHWATER INVESTIGATIONS 66b 



birds and winds. Barrois also notes for the fauna of the 

 Azores that the cosmopolitan European species which are 

 present are characterized by resting stages of some sort, indi- 

 cating thus passive introduction by birds, water insects and 

 also by man. Such a population must have come gradually to 

 the islands. 



The success with which plankton organisms may be trans- 

 ported during some stage in their life history is further evinced 

 by the ease with which some of these organisms can be raised 

 from dried mud, as done by Sars. In this dried condition such 

 forms may remain years without losing power of development 

 under satisfactory conditions of environment. The older ex- 

 periments on that point have recently been confirmed by At- 

 kinson. 



Among other agencies in dispersion must be noted the 

 glacial epoch and Voigt has followed out with great care the 

 effect upon the population of mountain brooks produced by the 

 glacial period and the present gradual supplanting of one spe- 

 cies by another. 



According to Garbini (94) the limnetic fauna is a passive 

 importation from northern centers of dispersion, and Strodt- 

 mann (96) finds its extended uniformity both in Europe and 

 North America evidence of a previous common center in 

 northern polar regions from which the limnetic fauna spread 

 southward. The marine forms in Lake Garda Garbini (94) 

 believes are not a fauna relicta but either active migrants from 

 the Adriatic in most part or in a few cases passive transports 

 from the northern ocean. 



Of especial interest on this question is the discussion con- 

 cerning the origin of the Nemertines, a purely marine group 

 with scattered freshwater species. According to Montgomery 

 (95a) these forms are of double origin, (1) direct migrants 

 through rivers to lakes and (2) relict forms in lakes. Their 

 recent origin is shown by variable structural features. Du 

 Plessis (95) would limit the former to rivers and to lakes never 

 occupied by the sea. Garbini (96a) thinks those of Lake 



