5. The Columbian.—This includes the coast drainage of British Columbia and Southeastern Alaska, the basins of the Fraser and Columbia rivers, the coastal part of the State of Washington, and the northern part of Idaho and Montana, west of the Selkirk Range and its more southern equivalents in the Eocky Mountain region. The northwestern extension of this system in Alaska, between the Coast Ranges and the sea, including the Alaska Peninsula and the Aleutian Islands, being really an extension northwestward of the Columbian system, might perhaps for convenience be called the Alaskan sub-region.
6. The Yukonian.—This system includes the entire drainage of the Yukon river, the tundra north of it and the basin of the Kuskokwim; or all of Alaska north, northwest and west of the Alaskan range, as well as that of the basin east of the Coast ranges drained by the Yukon and its tributaries.
We know through the researches of General G. K. Warren, IT. S. Engineers, that a considerable portion of the original Mississippi drainage near its former headwaters has, through changes of level of the earth near the 49 th parallel, been captured by the Hudsonian drainage system, and it is not unreasonable to suppose, that a certain proportion of the Mississippi fresh-water fauna was captured with the streams in which it lived. Some such supposition seems necessary to explain the far northern extension of certain Mississippi Valley species in the Hudsonian region, while other drainages equally suited to their inhabitation are destitute of them.
Another factor in the distribution of land as well as fresh water shells is the former existence of a continental ice-cap by which the entire mollusk fauna of the region occupied by ice would be exterminated; though, in the interglacial periods, the external fauna might advance as the ice retreated, only to be driven out again on the return of the ice. Such fluctuations have been well shown, by the researches of Dr. Coleman, of Toronto, to have occurred in the valley of the Don, near that city.
The vast territories included in these drainage systems are, it is true, only partially and imperfectly explored for mollusks. Yet certain portions of them are tolerably well known and the uniformity imposed on the fauna by its high northern position and unvaried conditions, leads to the belief that, while much is yet to be known in tracing out the details of distribution, little is to be expected in the way of absolutely new species, even from this immense territory yet to be explored.
It would be rash to conclude that nothing new remains to be found, but our expectations should certainly be moderate.
There are a few characteristic fresh-water forms in Greenland,
