I wish I had known of Clitoria Mariana-acuminata, etc., in time to add it to my list in the last number of “Silliman’s Journal;” a copy of the article was sent to Dr. Hooker by post last week. I will send more, from my extras, presently.
I am quite prepared for what you say about interchange of species of United States and Europe taking place via Asia, instead of across the Atlantic; but you will see there are a few, besides aquatics (Subularia, Eriocaulon, etc.), which would seem to have taken the shorter cut.
As respects identical species, interchange is the only thing that, on our views of what a species is, will explain the occurrence of the same species here and there. But as to genera, I do not yet feel free to assume an interchange, or a former continuity of land, between two widely separated regions on account of their having identical genera or closely related species. I see no reason why cognate species may not have been originally given to most widely separated stations; and, as to the facts of association, can we say more than this, that the species of a genus are apt to be confined to one part of the world? Are there not too many cases to the contrary to warrant our suspecting former continuity of two remote districts on account of common genera? Peculiar genera, such as Torreya, Illicium, Philadelphus, Astilbe, etc., divided between Japan and the United States of America, indicate some peculiar relation, and are most noteworthy, but I do not see why it points to connection.
I am very glad you are turning your good, logical mind and immense knowledge to this class of topics; but do not let it run off with too much of your valuable time. I take far more satisfaction in discussing questions of botanical affinity; and long to get back to that sort of work. Just now, I must needs be absorbed in elementary work and teaching, but look to see an end of this.
I have been watching the development of the ovules of Magnolia; nothing can be more normal than they are, in the early stages.
When Wright comes home from Cuba I expect to get hold of his considerable north Japan collection, which I expect to find very interesting on questions of distribution, the very questions you ask me to consider.
I doubt if our “mountain backbone” actually stops any species, itself, from advancing east or west.
I wish you would compare our White Birch with the European B. alba, and let me know the result. Also the Chestnuts....
TO R. W. CHURCH.
Cambridge, May 15, 1857.