334 HENRY b. ward: 



According to Scharff (95) the fauna of Ireland has come by 

 direct extension from Scotland and Wales, with which the con- 

 nection previously existing broke down during the pleistocene 

 period. Hanitsch (95) holds, however, that among the fresh- 

 water sponges three of the six species are American. The 

 importation occurred by means of gemmules borne on floating 

 pieces of wood in the Gulf Stream, or indirectly by mi- 

 grating birds through Greenland. The failure of these species 

 to spread further is explained on the ground of their inability 

 to compete with native species. 



Simroth (90) believes that a secondary adaptation of many 

 land plants and animals has followed upon their immigration 

 into fresh water. Such groups as Hydraclmids show clear evi- 

 dence of a land life. Fresh water has been furthermore a place 

 of refuge for many ancient forms such as the Ganoids, the 

 Dipnoi and the Branchiopoda. Here also Guppy. 



Beddard and Lankester show how tropical animals act- 

 ually are transported from place to place on aquatic plants. 

 Garbini (95b) has obtained positive evidence of passive trans- 

 portation of freshwater animals in that ten species representing 

 seven groups were actually collected in transit on mammals, 

 birds, amphibians and aquatic insects, thus evincing the impor- 

 tant role of these forms in the dispersion of the aquatic fauna. 

 Schewiakofl gards currents of air and water and 



actively migrating dbimals as the efficient means of distribu- 

 tion for protozoa. Their successful introduction depends ac- 

 cording to Franc. ; (97) not on meteorological conditions but on 

 hydrological surroundings and on associated plant forms. 

 Kofoid (96b) states briefly the agencies, human and meteoro- 

 logical, important in dispersion. 



The insufficiency of our knowledge and the impossibility of 

 drawing reliable conclusions regarding the distribution of 

 freshwater forms from the data at present on record are shown 

 by the statements of Hartwig (98) concerning rare Entomos- 

 traca and Jennings (98) on a supposed Asiatic rotifer. 



The fauna of island lakes is believed by Richard (98) in the 

 case of the Canary Islands to be introduced in the egg stage by 



