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 numerou