Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/395

382 —but by ground-feeders and littoral forms. Sir John Richardson, in his admirable 'Report on the Zoology of North America,' observed a similar peculiarity in the distribution of the vertebrate animals common to North America and Europe. Writing of the fishes of the order Malacopterygii sub-Brachiales, he remarks, "Most of the fish of this order feed on or near the bottom, and a very considerable number of the species are common to both sides of the Atlantic, particularly in the higher latitudes, where they abound. It does not appear that their general diffusion ought to be attributed to migration from their native haunts, but rather that in this respect they are analogous to the owls, which, though mostly stationary birds, yet include a greater proportion of species common to the old and new worlds than even the most migratory families. Several of the Scomberoideæ, which feed on the surface, have been previously noted as traversing many degrees of longitude in the Atlantic; but the existence of the ground-feeding Gadoideæ, in very distant localities, must be attributed to a different cause, as it is not probable that any of them wander out of soundings or ever approach the mid-seas" (p. 218). This is as true of the mollusks as of the fish, and, doubtless, the cause is one and the same in each case. That cause must be sought for in the conditions presented by the North Atlantic and Arctic Seas during the glacial epoch. Since that period there appears to have been no interchange of species between the Northern Seas of the old and new world, so far as mollusks are concerned. New forms have arisen among the old ones, created to derive the bene- fits of new conditions; strangers have migrated along the coasts from the south on both sides of the Atlantic—their fry, during their natatory condition, availing themselves of the transporting powers of favouring currents; old species have returned to colder climes and limited their range, or in some cases have altogether disappeared; but not a single littoral or coast-inhabiting mollusk has found its way across the Atlantic in either direction since that ancient time, anterior to all human records, and probably long anterior to the appearance of man on our earth, when an Arctic Sea, inhabited by a limited and uniform fauna, extended from the then western coasts of Siberia into the heart of North America, and southwards in Europe to the parallel of the Severn, and in America to near that of the Ohio. This uniformity of fauna, this diffusion of littoral and nonmigratory forms, must have owed its existence to uniformity of conditions not now met with within that area. There could not then have been such a separating abyss between Northern Europe and Boreal America as now divides them; the sea, through a great part,