the N. and S. I have expressed this
very obscurely, but you will understand, I think, what I mean. It is
a parallel case (but with a greater difference) to the species of the
mountains of S. Europe compared with the arctic plants, the S. European
alpine species having been isolated for a longer period than on the
arctic islands. Whether there are many tolerably close species in the
warm-temperate lands of the S. and N. I know not; as in La Plata, Cape
of Good Hope, and S. Australia compared to the North, I know not. I
presume it would be very difficult to test this, but perhaps you will
keep it a little before your mind, for your argument strikes me as
by far the most serious difficulty which has occurred to me. All your
criticisms and approvals are in simple truth invaluable to me. I fancy I
am right in speaking in this note of the species in common to N. and S.
as being rather sub-arctic than arctic.

This letter does not require any answer. I have written it to
ease myself, and to get you just to bear your argument, under the
modification point of view, in mind. I have had this morning a most
cruel stab in the side on my notion of the distribution of mammals in
relation to soundings.

LETTER 335. J.D. HOOKER TO CHARLES DARWIN. Kew, Sunday [November 1856].

I write only to say that I entirely appreciate your answer to my
objection on the score of the comparative rareness of Northern
warm-temperate forms in the Southern hemisphere. You certainly have
wriggled out of it by getting them more time to change, but as you must
admit that the distance traversed is not so great as the arctics have to
travel, and the extremes of modifying cause not so great as the arctics
undergo, the result should be considerably modified thereby. Thus: the
sub-arctics have (1) to travel twice as far, (2) taking twice the time,
(3) undergoing many more disturbing influences.

All this you have to meet by giving the North temperate forms simply
more time. I think this will hardly hold water.

LETTER 336. TO J.D. HOOKER. Down, November 18th [1856].

Many thanks for your note received this morning; and now for another
"wriggle." According to my notions, the sub-arctic species would advance
in a body, advancing so as to keep climate nearly the same; and as long
as they did this I do not believe there would be any tendency to change,
but only when the few got amongst foreign associates. When the tropical
species retreated as far as they c