SciiARFF — Former Lund- Bridge between N. Europe and N. America. 21 



have expressed their agreement with this theory. Mr. Madison Grant believes 

 that the distribution of tlie living fauna points to the existence of continuous 

 land between Greenland, Spitsbergen, and Scandinavia in Pleistocene times.' 

 Prof. Lobley'' assumes a land-bridge in Pre-Glacial times, extending from 

 Europe to Iceland, Greenland, and Labrador; whereas Prof. Jacobi advances a 

 similar theory in less definite terms, for he speaks only of a long-continued 

 land-connexion between the New and the Old World by way of Greenland.^ 

 Dr. Arldt considers the land-bridge to have existed since the Oligocene 

 period, and to have been finally destroyed in Pliocene times.* 



Of all the theories which have been advanced in explanation of the 

 occurrence of identical species on both sides of tlie Atlantic Ocean, only the 

 following three have met with wide approval :— 



1. Migration from Europe across Asia and a Bering Strait land-bridge 



to America or vice versa. 



2. Occasional transport by birds across the Atlantic Ocean, 

 o. Migration across a direct Atlantic land-connexion. 



If we consider the zoological evidence alone, namely, the absence of 

 Helix horfensis from Asia and Western America, the distribution of the 

 Perches and the freshwater Pearl Mussel, and that of the freshwater Sponges, 

 the first of the three hypotheses is scarcely applicable to these instances of 

 distribution, and does not therefore explain the presence of identical species 

 on both sides of the Atlantic in a satisfactory manner. 



As regards the supposed conveyance by birds of seeds and invertebrates 

 across the same ocean, the second theory must be applicable to a transport 

 in two directions, both from America to Europe as well as vice versa. 



Mr. Eagle Clarke, of the Edinburgh Museum, who has made a special 

 study of bird migration, informs me that in his opinion all the American 

 species of birds that have made their appearance in Europe have travelled by 

 way of Greenland and Iceland. All of them, he says, are birds of high 

 northern summer range in America ; and they are mostly birds of the year 

 which, instead of returning southward or westward in their autumnal flights, 

 have taken an eastward course. All the other accidental visitors from 

 America, he thinks, must have had an assisted passage across tlie ocean as 

 cage-birds. There is only one point which I venture to think Mr. Clarke 

 may have overlooked, namely, the possibility of some American birds having 



' Grant, Madison, " Origin of Mammals of North America," p. 12. 



- Lobli-y, J. L., " American Faiiua ami ils Origin," p. 27. 



■' Jacobi, A., " Lage und Form biogcographischi-T Gebiete," p. 'iOi. 



' Arldt, Tb., "Butwicldungd. Kouliuoute," p. 406. 



