Page:Popular Science Monthly Volume 66.djvu/369

Rh modify essentially the general rule, and like the valleys themselves, these species disappear a short distance north of the 49th parallel. I was able to show, some thirty-five years ago, that in a broad way the distribution of the birds and plants presents analogous features.

The land shell fauna of the northeast extremity of Asia has little individuality, but represents a mingling of the depauperated extremes of the faunas of northeast China and of Europe with that series of species which is sometimes called the circumpolar or circumboreal fauna.

Much of the apparent poverty of the fauna may be due to insufficient collecting, but even when the most generous allowance for this factor is made, it still remains certain that the molluscan population is far less in variety than might reasonably be expected.

The palaearctic fauna of Europe appears to extend clear across Northern Asia, losing a large proportion of its species on the way, until (if the circumboreal species be excluded) only about thirty species reach the headwaters of the Lena and the barrier of the Stanovoi range. A very remarkable local fauna exists in the great 'Relictensee' of Siberia, Lake Baikal, but it does not appear to have tinctured the East Siberian fresh-water fauna outside of that lake, to any appreciable extent. It is possible that the comparatively recent emergence of a large part of Eastern Siberia from the sea, and the presence of the vast desert region to the south and west, may enter into the explanation of this sparse shell-fauna, as well as of some of the peculiarities of the Baikal faunula.

Southeast of the Stanovoi range we find between the mountains and the sea, the valley of the Amur and several smaller valleys, such as the drainage-basins of the Ud and the Tugar. To the southwest the sources of the Amur emerge from the deserts of Gobi and Dauria, and along the line of these water courses has crept a certain number of molluscan forms intimately related to or identical with those of Mongolia, China and the Orient. This forms the second element of the fauna of Northeast Siberia.

The number of purely endemic species is remarkably small and a portion of those claimed to be of this character are probably mere local mutations of widespread palearctic forms already known. Yet it would seem as if a more thorough exploration must add largely to the species now known, and it is almost incredible that the luxuriant fertile valleys of Kamchatka and the innumerable streams and lakes of that country should not be well populated with mollusks.

There are a few species which seem to be common to the shores of Bering Sea, both Asiatic and American, such as Succinea chrysis, Pyramidula pauper, Punctum conspectum, and Anodonta beringiana.