126 The Irish Naturalist. 



we be obliged to accept Wallace's theory as to a more or less 

 uniform fauna and flora once spreading over the whole north 

 temperate zone. In any case, it is significant that of American 

 forms, with the exception of a few plants, only such animals 

 have been found in Ireland as might easily have migrated 

 there. Three agents may have served in carrying sponge- 

 gemmules from North America across to Ireland — winds, 

 ocean currents, and birds. Strong winds might carry dried 

 gemmules almost any distance, like plant-seeds, and the 

 position of Ireland, together with its western winds favours 

 such a possibilit}^ The Gulf Stream might have carried 

 gemmules or even entire Sponges containing gemmules, loose 

 or attached to floating timber, from North American rivers 

 to Ireland. When once arrived on the Irish shore their 

 further dispersal to higher levels must have been a com- 

 paratively simple matter. Similar cases must have happened 

 often enough. Fresh- water Sponges, if they had, as we 

 suppose, their ancestors in marine forms, must in any case 

 have travelled inland and to higher levels. However, I do 

 not know for how long a period gemmules can stand entire, 

 or partial immersion in sea-water. Finally, we may look to 

 birds as agents in the dispersal of gemmules from N. 

 America to Ireland. Wallace (11, vol. I., p. i6) says that 

 * small and weak birds are often carried accidentally across 

 great wadths of ocean by violent gales.' — * No less than sixty- 

 nine species of American birds have occurred in Europe, 

 most of them in Britain and Heligoland.' Such birds would 

 naturally first alight at the west coast ot Ireland, and would 

 be more liable to leave any gemmules there than at sub- 

 sequent resting-places. Again, a number of migratory birds, 

 common to Europe and America, regularly visit Greenland 

 (Wallace, 11, vol. II., p. 138). It is possible that, even by such 

 roundabout methods, gemmules could be carried from America 

 to Greenland and thence to Ireland. A similar communica- 

 tion via Iceland seems less probable, as, although there are no 

 less than forty species of annual visitants from Europe to 

 Iceland, there seems to be no regular inter-communication 

 between N. America and Iceland (Wallace, 11, vol. I., p. 198). 

 The explanation of the fact that these Sponges, once arrived 

 in the west of Ireland, did not spread out further east, is 

 perhaps that competition was too severe. 



