I must say, that the chart seems to me against land-extension explaining the introduction of organic beings. Madeira, the Canaries and Azores are so tied together, that I should have thought they ought to have been connected by some bank, if changes of level had been connected with their organic relation. The Azores ought, too, to have shown more connection with America. I had sometimes speculated whether icebergs could account for the greater number of European plants and their more northern character on the Azores, compared with Madeira; but it seems dangerous until boulders are found there. (327/2. See "Life and Letters," II., page 112, for a letter (April 26th, 1858) in which Darwin exults over the discovery of boulders on the Azores and the fulfilment of the prophecy, which he was characteristically half inclined to ascribe to Lyell.)
One of the more curious points in Maury is, as it strikes me, in the little change which about 9,000 feet of sudden elevation would make in the continent visible, and what a prodigious change 9,000 feet subsidence would make! Is the difference due to denudation during elevation? Certainly 12,000 feet elevation would make a prodigious change. I have just been quoting you in my essay on ice carrying seeds in the southern hemisphere, but this will not do in all the cases. I have had a week of such hard labour in getting up the relations of all the Antarctic flora from Hooker's admirable works. Oddly enough, I have just finished in great detail, giving evidence of coolness in tropical regions during the Glacial epoch, and the consequent migration of organisms through the tropics. There are a good many difficulties, but upon the whole it explains much. This has been a favourite notion with me, almost since I wrote on erratic boulders of the south. It harmonises with the modification of species; and without admitting this awful postulate, the Glacial epoch in the south and tropics does not work in well. About Atlantis, I doubt whether the Canary Islands are as much more related to the continent as they ought to be, if formerly connected by continuous land.
Hooker, with whom I have formerly discussed the notion of the world or great belts of it having been cooler, though he at first saw great difficulties (and difficulties there are great enough), I think is much inclined to adopt the idea. With modification of specific forms it explains some wondrous odd facts in distribution.
But I shall never stop if I get on this subject, on which I have been at work, sometimes in triumph, sometimes in despair, for the last month.
LETTER 328. ASA GRAY TO CHARLES DARWIN. Received August 20th, 1856.
I enclose you a proof of the last page, that you may see what our flora amounts to. The genera of the Cryptogams (Ferns down to Hepaticae) are illustrated in fourteen crowded plates. So that the volume has become rather formidable as a class-book, which it is intended for.
I have revised the last proofs to-day. The publishers will bring it out some time in August. Meanwhile, I am going to have a little holiday, which I have earned, little as I can spare the time for it. And my wife and I start on Friday to visit my mother and friends in West New York, and on our way back I will look in upon the scientific meeting at Albany on the 20th inst., or later, just to meet some old friends there.
Why could not you come over, on the urgent invitation given to European savans--and free passage provided back and forth in the steamers? Yet I believe nobody is coming. Will you not come next year, if a special invitation is sent you on the same terms?
Boott lately sent me your photograph, which (though not a very perfect one) I am well pleased to have...
But there is another question in your last letter--one about which a person can only give an impression--and my impression is that, speaking of plants of a well-known flora, what we call intermediate varieties are generally less numerous in individuals than the two states which they connect.