To J. D. Hooker   15 November [1856]¹

My dear Hooker

I shall not consider all your notes on my M.S. for some weeks, till I have done with crossing; but I have not been able to stop myself meditating on your powerful objection to mundane cold period, viz that many fold more of the warm-temperate species ought to have crossed the Tropics that of the sub-arctic forms.—² I really think that to those who deny modification of species, this would absolutely disprove my theory. But according to the notions which I am testing, viz that species do become changed & that time is a most important element (which I think I shall be able to show very clearly is the case) in such change, I think the result would be as follows. Some of the warm-temperate forms would penetrate the Tropics long before the subartic, & some might get across the Equator long before the sub-arctic forms could do so, (ie always supposing that the cold came on slowly) & therefore they must have been exposed to new associates & new conditions much longer than the sub-arctic. Hence I sh^d. infer that we ought to have in warm temperate S. hemisphere more representative or modified forms, & fewer identical species than in comparing the colder regions of the N. & S.— I have expressed this very obscurely, but you will understand, I think, what I mean.— It is a parallel case, (but with greater difference) to the species of the Mountains of S. Europe compared with the artic 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 warmer temperate lands of the S. & N. I know not; as in La Plata, C. of Good Hope, & 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 critisms & 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. & S as being rather sub-arctic than arctic.—

This letter does not require any answer: I have written it to ease myself & 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 distribution of Mammals in relation to soundings.³

My dear Hooker | Yours most truly | C. Darwin