Land-bridge from Northern Europe to North America. 38 
land-connexion to the north of the Atlantic Ocean—a belief already 
upheld by so much evidenee, both geological and zoological.”’ ! 
Quite a number of naturalists believe that any resemblance between 
the European and the American fauna must have arisen, not from any 
direct intercourse between Europe and America, but by a migration 
across Asia and a Bering Strait land-connexion. The supposition of 
an ancient northern Pacific land-bridge presents fewer difficulties 
to them than the Atlantic one, and is preferred for that reason. 
Dr. Horvath, for example, who states that no less than 128 species of 
Hemiptera are common to the two continents, argues that they all 
must have crossed Asia in reaching the one from the other.? But he 
and those who agree with him were apparently unaware that certain 
freshwater species common to Kurope and America are almost totally 
absent from Asia or Western America. 
Let us take, for example, our common Perch (Perca fluviatilis), 
a variety of which also inhabits North America. 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 
Europe. 
Another example that I have had occasion to quote in my work on 
European Animals (p. 35) is the freshwater Pearl-Mussel (Deleagrina 
margaritifera). On our continent it inhabits the British Islands except 
Kastern England, the mountain streams of Scandinavia, and the hill- 
region of Central Europe except the Alps. Far to the east it reappears 
in a different form in the River Amur in Eastern Siberia, in the 
island of Sakhalin, and in Kamchatka. 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 Eastern North 
America and in Central and North-Western Europe. America is 
undoubtedly its original home. From it the mussel 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 
between the two continents explains their distribution certainly better 
than any other theory. 
The most striking piece of evidence 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. 
A western species in Europe, Helix hortensis, is remarkable for its 
extensive northern range. It occurs in Scandinavia, all over the 
British Islands, in the Shetlands and Farées, and even in Iceland. 
It is altogether absent from Asia. Its occurrence in Southern 
1G. H. Carpenter, ‘‘ Collembola from Franz Joseph Land’’: Proc. R. Dublin 
Soc., 1900, ix, p. 276. cS 
2 G. Horvath, ‘‘ Faunes hémiptérologiques’’: Ann. Hist. Nat. Mus. Hungarici, 
1908, iv, pp. 4-7. 
DECADE V.—VOL. VII.—NO. I. 3 
