shown in our table for the Pliocene may very easily be traced to the effect of land-ice. Let us turn to the little map (Fig. 16) which Arldt has given for, the distribution of those organisms which appeared to him to be most decisive to the question of the North Atlantic bridge. The recent genera of earth-worms of the Lumbricidæ are, as already mentioned, distributed from Japan to Spain, but on the other side of the ocean only in the east of the United States. The pearl-mussel occurs on the line of fracture of the continents in Ireland and Newfoundland, and adjacent areas on both sides.
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Fig. 16.—Distribution of North Atlantic organisms, after Arldt. Dotted line: garden snails. Dashes:—Lumbricus—earth-worms. Dots and dashes: perches. Shaded N.E.—S.W.: pearl-mussel. Shaded N.W.—S.E.: mud-minnows (Umbra).
The distribution of garden snails from South Germany across the British Isles, Iceland and Greenland to the American side, where they are only found in Labrador, Newfoundland and the Eastern States is still more striking. This also applies to the family of the perches (Percidæ), and other fresh-water fishes. Possibly the common heather (Calluna vulgaris), which outside Europe is only to be found in Newfoundland and the neighbouring districts, might be mentioned, and also, conversely, that many American plants are quite confined in Europe to the west of Ireland. Even if the Gulf Stream can possibly be drawn in as the explanation of the latter, this cannot be done in the case of the heather. Many things testify
