Page:Popular Science Monthly Volume 1.djvu/743
ON THE DERIVATION OF AMERICAN PLANTS.
The larger part of the genera of our own region which I have enumerated as wanting in California are present in Japan or Mantchooria, along with many other peculiar plants divided between the two. There are plants enough of the one region which have no representatives in the other. There are types which appear to have reached the Atlantic States from the South, and there is a larger infusion of subtropical Asiatic types into temperate China and Japan; among these there is no relationship between the two countries to speak of. There are also, as I have already said, no small number of genera and some species, which, being common all round or partially round the northern temperate zone, have no special significance because of their occurrence in these two antipodal floras, although they have testimony to bear upon the general question of geographical distribution. The point to be remarked is that a very large proportion of the genera and species which are peculiar to North America as compared with Europe, and largely peculiar to Atlantic North America as compared with the California region, are also represented in Japan and Mantchooria, either by identical or by closely-similar forms. The same rule holds on a more northward line, although not so strikingly. If we compare the plants, say of New England and Pennsylvania (latitude 45° 47'), with those of Oregon, and then with those of Northeast Asia, we shall find many of our own curiously represented in the latter, while only a small number of them can be traced along the route even so far as the western slope of the Rocky Mountains. And these repositories of Eastern- American types in Japan and neighboring districts are in all degrees if likewise. Sometimes the one is undistinguishable from the other; sometimes there is a difference of as great but hardly of as tangible character; sometimes the two would be termed marked varieties if they grew naturally in the same forest, or in the same region; sometimes they are what the botanists call representative species, the one answering closely to the other, but with some differences regarded as specific; sometimes the two are nearly of the same genus or not quite that, but of a single or very few species in each country, when the point which interests us is that this peculiar limited type should occur in two antipodal places and nowhere else. It would be tedious, and, except to botanists, abstruse, to enumerate instances, yet the whole strength of the case depends upon the number of such instances. I propose, therefore, if the Association does me the honor to print this discourse, to append in a note a list of the more remarkable ones. But I would mention two or three cases as specimens. Our Rhus toxicodendron, or poison-ivy, is exactly repeated in Japan, but is found in no other part of the world, although a species like it abounds in California. Our other species of Rhus (R. venenata), commonly called poison-dogwood, is in no way represented in Western America, but has so close an alliance in Japan that the two were taken for the same by Thunberg and Linnæus, who called them both R, vernix. Our