18 Proceedings of ihc Royal Irish Academy. 



of which also inhabits North America (fig. o). It is absent not only from a 

 large part of Asia, but also from western North America. Certainly this 

 looks like a case of direct migration from America to Emope. Kevertheless, 

 those who favour accidental methods of dispersal may claim that the 

 feet and feathers of birds had some share in this distribution ; for it is assumed 

 that fish-spa%m may occasionally cling to birds alighting on water. It will 

 scarcely be conceded, however, that birds have the power to select the 

 spawn of Perch, and carry it across the Atlantic to the exclusion of that of 

 all other species. For according to Mr. Tate K^an, to whom I am indebted 

 for the map of distribution which I herewith reproduce, the Perch and the 

 whole Perch family {Pcrcida!) are absent from western North America and 

 largely from eastern Asia. An accidental transport of freshwater fish-eggs 

 from land to land across the Atlantic, either by wind or waves, seems to me 

 quite beyond the range of possibility. Human introduction is altogether out 

 of the question, l>ecause, apart from the Common Perch, we have to deal with 

 genera and species of Percidae found in the one continent and not in the 

 other. These must have evolved from some remote ancestor, common to 

 America and Europe, long anterior to the appearance of Man. 



Another example that I have had occasion to quote in my work on 

 " European Animals " (p. 35) is the freshwater Pearl-Mussel {Margariiana 

 maryaritifer). On our continent it inhabits the British Islands except 

 eastern England, the mountain streams of Scandinavia, and the hill 

 region of Central Eurof* except the Alp.s. Far to the east it reappears in 

 a different fonn in the River Amur in eastern Siberia, in the island of 

 Sakhalin, and in Kamtchatka. Another variety is met with across the 

 Bering Strait in Alaska and in western North America generally. The type 

 form occurs in the Quebec province of Canada, in the Lower Saskatchewan 

 River, and in New England. The typical freshwater Pearl Mussel is only 

 met with in ea-stem North America and in C€ntral and north-western 

 Europe. America is undoubte<lly its original home. From it the mus.sel 

 spread to Europe in an eastward direction, and not by way of Asia. As the 

 fry of these mussels attach themselves to the gills of fishes, they are liable to 

 wide dispersal within at least one river-system ; but fishes in this case could 

 scarcely have aided them in reaching Europe. A land-connexion l>etween the 

 two continents explains their distribution certainly better than any other 

 theory. 



The most striking piece of e%idence we possess in favour of a Pre-Glacial 

 land-connexion between north-western Europe and north-eastern North 

 America is the presence in the latter country of the snail Helix hortensis. 



