Dear Mr Darwin—

Your very kind note of 13 August last—reached me only very lately when I returned for a few days to England—¹ I need not say that I read it with the greatest interest. I feel as strongly as possible that the hypothesis that I have ventured to put forward requires very much fuller development than I have yet been able to give it before it can claim even that degree of assent which belongs to a conclusion towards which several separate lines of probability appear to converge. I need not say that I have no notion of endeavouring to lure you into controversy but as you have been good enough to notice some specific difficulties I will venture to make a few observations on them²

1. Absence of experimental proof that the higher plants are more intolerant of carbonic acid gas than the lower—

I believe that all the experiments hitherto made on the cultivation of plants in an atmosphere surcharged with CO₂ have been inconclusive owing to the practical difficulty of maintaining a nearly uniform composition in the imprisoned air whose CO₂ is rapidly decomposed by the growing plants—but even if it should be shown that there is no such difference as has been supposed in the effect of excessive CO₂ on the growth of the higher & lower plants I should not regard that as very important to my argument. It seems to me probable that the great & important difference between the condition of the lowlands & the high mountains in palæozoic (pre-coalmeasure) times was not so much the chemical difference between the effect of the less charged air of the upper region & that of the more charged air of the lower zone as the physical difference in the effect on vegetation of rapid & considerable changes of temperature & seasons of activity & repose, which would be felt in the more freely radiating upper region as compared with the lower zone where difference between night & day w^d be almost insensible & that of the seasons comparatively slight.

2. In considering the question of the probable degree of diffusion of CO₂—in an atmosphere containing 20 times the present proportion—(25 times would be I think nearer the mark) it would be well to consider that the only important interference with the law which w^d regulate an atmosphere at rest—(whose condition I had calculated) arises from the action of winds— But the diminished radiation from the greater part of the surface of the planet when surrounded by an atmosphere surcharged with CO₂ & aqueous vapour, would cut off or nearly so at their source most of the aerial currents & very much lessen the cause of disturbance.

3 Of course under almost any reasonable view of the origin of the existing flora one would expect a certain amount of relation between the floras of the mountain masses & that of the surrounding lower regions— I will not presume to give an opinion whether on the whole the facts favour the idea of the mountain plants being derived from the low country or vice versa— I should have thought that both were true to some extent— But apart from this I should think that the number of genera & species peculiar to the high mountains is great enough to make an a priori probability in favor of the opinion that their diffusion must date from a geologically remote period   I stop myself lest I should contrary to my intention let this degenerate into an argument—instead of merely thanking you for your remarks   If I am able to carry on the work that I have in my head I shall hope to obtain more countenance for my ideas than I can now expect from you

believe me most sincerely yours | John Ball

C. Darwin Esq