To C. J. F. Bunbury   21 April [1856]¹

My dear Bunbury

You are quite right, I do take a very great interest about the Cape Flora & Fauna, & I thank you much for your letter,² which, as all yours do, has pleased & instructed me much.— I have lately been especially attending to Geograph. Distrib, & most splendid sport it is,—a grand game of chess with the world for a Board. The fact you allude to about the zoology (at least mammifers) of the Cape not being nearly so peculiar as the Botany has often struck me much: I think the most probable hypothetical explanation is that it was long a group of islands, since united with the continent allowing the vertebrata to enter.—

Thank you about the Colletia, I called on Lindley, but c^d. extract nothing & wrote to the Gardener who raised the seed, (but have not, & shall not receive any answer) to ask whether he ever had seed from S. America of any kind; undoubtedly the common form was in the Garden.³

I am very glad to hear you are still thinking of Madeira; there seems to me much to be done there yet; but I hear from M^r. Lowe, he is going to publish a Flora, & he has sent me a curious account of vegetation of P. Santo.⁴ A careful comparison of the Floras of Madeira, Azores, & Canary Is^d. would, I cannot doubt, lead to some very curious results.

You speak in far too flattering a way about my work, in which I will persevere; & I will endeavour (eheu how difficult) “to be cautious & candid & avoid dogmatism”. My determination to put difficulties, as far as I can see them, on both sides is a great aid towards candour; because I console myself, when finding some great difficulty, in endeavouring to put is as forcibly as I can.— I am trying many little experiments, but they are hardly worth telling, though some I am sure will bear on distribution & I think on aquatic plants.⁵

As you say you like scientific chat, & your kind letter makes me sure that you will not think me an egotistical bore, I will tell you of a theory I am maturing (by the way please do not mention it to anyone, for 2 directly opposite reasons, viz whether valueless or valuable). As glacial action extended over whole of Europe, & in Himmalaya, on both sides of N. America & both sides of Southern S. America & I believe in N. Zealand, within very late times (existence of recent species); I cannot but think the whole world must have been rather colder during the Glacial Epoch: (I know I ought to be able to show that the glacial action was actually & absolutely coincident in North & South, & this I cannot do, nor can I here enter in details to show how far I can show them coincident)⁶

At this period I look at the intertropical plants as somewhat distressed, but not (or only a few) exterminated.— Under these conditions I consider it probable that some of the warmer temperate plants would spread into the Tropics, whilst the arctic plants reached the foot of the Alps & Pyrenees. (according to poor Forbes’ view; by the way I had this part of the theory written out., 4 years before Forbes published!)⁷

Some, I consider it possible might cross the Tropics & survive at C. of Good Hope, T. del Fuego & S. Australia; but within the Tropics, when warmth returned, all would be exterminated, except such as crawled up mountains, as in Ceylon, Neilgherries, Java, Organ mountains in Brazil. This theory, I conceive, explains certain aquatic productions in S. hemisphere &c &c. (& European Fish at C. of Good Hope)— But on the view that species change, it throws, I think, far more light on the analogous, but not identical species, on the summits of the above named mountains. Of course I cannot enter in details (& you would not care to hear them) on this subject, which I am sure in some degree would render the view more probable than it will seem to you at first.—

You will probably object, why have so many more Northern species & forms gone to the south, than southern forms come to the north; I can explain this only on a pure hypothesis of cold having come on first from the north; but there has been some migration from south to north, as of Australian forms on Mountains of Borneo. And I am sure I have notes of a few S. African forms, as wanderers across the Tropics, into N. Africa & Europe: is not this so with Gladiolus, Stapelia(?). Can you help me in this, either identical species, or allied forms, of well marked S. African forms? By the way I look at Abyssinia, during the cold period, as the channel of communication; for some, (as I know from Richard) very northern temperate species of plants are found there; & some S. African forms likewise.—⁸

There, I am sure, you will agree that I have prosed enough on my own doctrines; which I may have to give up, but I strongly suspect that the theory is a sound vessel & will hold water. I look at the vegetation of the Tropics, during the cold period, as having been somewhat like the vegetation described by Hooker at foot of Himmalaya, as essentially Tropical, but with an odd mixture of Temperate forms & even identical species, before they became mostly modified.—

What will you say to such a dose of speculation! You will exclaim, “he is a pretty fellow to talk of caution”!—

Pray believe me | Your’s very sincerely | Charles Darwin

If at any time you are inclined to write pray attack my doctrine.—⁹

With respect to diffusion of water plants in very distant regions, it seems, as far as my doctrine is concerned, sufficient answer that the same species of water plants in the same continent are very widely diffused, & whatever the means of diffusion may be, the same means w^d. tend to carry them to the most distant parts during the cold period.— The same argument is applicable to the Glumaceæ to some extent; but Decandolle thinks that certain lowly organised phanerogams, which are very widely diffused (I forget whether he includes Glumaceæ, which I think some authors consider the highest of the monocots?) are diffused owing to such species having been very anciently created & therefore having had more time to become diffused.¹⁰ I doubt whether he has any grounds for his belief, without it be a very feeble analogy of the greater duration of mammifers compared with Molluscs.—