the
temperature of tropical Africa had been so recently lowered, would
certainly have spread northwards, and on the return of the heat
could hardly have been driven back into the sharply defined and very
restricted area in which it now exists.

As to the migration of plants from mountain to mountain not being so
probable as to remote islands, I think that is fully counterbalanced by
two considerations:--

a. The area and abundance of the mountain stations along such a range
as the Andes are immensely greater than those of the islands in the N.
Atlantic, for example.

b. The temporary occupation of mountain stations by migrating plants
(which I think I have shown to be probable) renders time a much more
important element in increasing the number and variety of the plants so
dispersed than in the case of islands, where the flora soon acquires
a fixed and endemic character, and where the number of species is
necessarily limited.

No doubt direct evidence of seeds being carried great distances through
the air is wanted, but I am afraid can hardly be obtained. Yet I feel
the greatest confidence that they are so carried. Take, for instance,
the two peculiar orchids of the Azores (Habenaria sp.) What other mode
of transit is conceivable? The whole subject is one of great difficulty,
but I hope my chapter may call attention to a hitherto neglected factor
in the distribution of plants.

Your references to the Mauritius literature are very interesting, and
will be useful to me; and I again thank you for your valuable remarks.

LETTER 397. TO J.D. HOOKER.

(397/1. The following letters were written to Sir J.D. Hooker when he
was preparing his Address as President of the Geographical Section of
the British Association at its fiftieth meeting, at York. The second
letter (August 12th) refers to an earlier letter of August 6th,
published in "Life and Letters," III., page 246.)

4, Bryanston Street, W., Saturday, 26th [February, 1881].

I should think that you might make a very interesting address on
Geographical Distribution. Could you give a little history of the
subject. I, for one, should like to read such history in petto; but I
can see one very great difficulty--that you yourself ought to figure
most prominently in it; and this you would not do, for you are just the
man to treat yourself in a dishonourable manner. I should very much like
to see you discuss some of Wallace's views, especially his ignoring
the all-pow