of Europe and North America. 207 



in regard to distribution, &c., which have been accumulated 

 by American naturalists during the last thirty years. Con- 

 sequently that work is far from being a good " standard of 

 comparison." To make a just comparison, all the shells on 

 our coast, from Labrador to Florida, should be compared with 

 those of Europe. 



And without going into a long discussion of his peculiar 

 views on the geographical distribution of our shells, I would 

 remark that, to an, American, it seems rather singular that 

 most European writers, whether zoologists or botanists, find it 

 necessary to trace back to a European origin all the existing 

 species of this country, and to suppose that they have 

 "migrated" from Europe to America and other countries in 

 spite of opposing currents and all other obstacles. Thus Mr. 

 Jeffreys can imagine that our land and freshwater shells could 

 have migrated from Europe all the way across Asia, the Pacific 

 Ocean and North America in order to reach Canada and New- 

 England; but he does not seem to think it possible that they may 

 have originated in America, and thence crossed to Europe 

 in the direction of the prevailing currents and winds. Never- 

 theless geology teaches us that America was a great continent, 

 in very early ages, when Europe was only a group of islands, 

 that no other country is richer in the remains of terrestrial ani- 

 mals and plants connecting the Tertiary and Cretaceous ages 

 with the present, that many of these supposed European forms 

 (whether terrestrial or marine) can be traced back into our Ter- 

 tiary formations quite as far (if not further) than tliey can in 

 Europe, and that many of the genera of animals, and especially 

 of plants, now found living in both countries can be traced back 

 to the Cretaceous in America and only to the Tertiary in 

 Europe. Moreover the great number and diversity of the 

 land and freshwater shells of America (e. g. of Unionida?, 

 3felania', &c.), and the peculiar facts in their geographical 

 distribution, cannot but convince any one familiar with the 

 subject that they have originated in America at a very remote 

 period ; which is confirmed by the fact that many of these can 

 be traced far back into our Tertiary formations. Nor are there 

 sufficient reasons for supposing that those of our species living 

 also in Europe have had a history different from those that are 

 still peculiar to America. 



Of course no one wil 1 deny that certain species of laud-shells 

 have been introduced from Europe in modern times by human 

 agency ; but, so far as most of the identical species are con- 

 cerned, it seems to us far more probable that America gave tliem 

 to Euro])e, rather than the contrary, and this whether animals 

 or plants, terrestrial or marine. 



