My dear Darwin,

I hardly know how to account for my long silence, after the very interesting letter which I received from you upwards of two months ago, & which I assure you I value highly.¹ But in truth I felt that I had nothing to say which could be at all an adequate return for your letter, & I waited (rather vainly) in hopes of “something turning up”, that is, in hopes of striking out some remark which might be worth sending.

About the Colletia, however, I have something to say.² I wrote to Joseph Hooker about it, & he wrote in reply:—“You are quite right about the Colletia; it was a hasty affair of Lindley’s, & would have gone beyond even my notions of variability.” I presume he means that Lindley was misled as to the garden origin of the supposed variety, or too hasty in believing it.

I am exceedingly interested by all you tell me about your researches & speculations on species & variation & distribution, & am delighted that you are going on working at the subject. I trust that you will not on any account give up the idea of publishing your views upon it; tho’ neither you nor any one else may be able to unravel the whole mystery, or to command the universal assent of naturalists, still the researches of one who has studied the whole question so long, & with such extensive knowledge & in so philosophical a spirit, cannot fail to be of very great advantage to science. The whole subject,—I mean every thing connected with the geography of plants & animals, including all the questions of distribution & variation, is to me particularly interesting & delightful; but how much we have yet to learn upon it! The difficulties which appear to attend upon each & every one of the theories,—of specific centres, of multiple creation, & of transmutation,—are so many, that what is most clear to me is the necessity of caution & candour, of avoiding dogmatism, & of giving a fair consideration to every fact & argument on any side. ³ I say this, because the theory to which you lean is the most remote from that to which I incline, & yet I am quite ready to admit that your notion may be the right one. Certainly, on the “specific centre” theory, it is very difficult to account for the fact that (to take one instance only,) certain plants, not in the least likely to have been introduced by man or by accident, are common to Europe & the Cape, & not found (as far as we know) in tropical Africa; such are Typha latifolia, Scirpus lacustris, & Scirpus fluitans. To be sure, one might cut the knot boldly, by saying that they may exist in the intermediate countries, tho’ not yet found there; but this would not be very satisfactory. How can one account, by the way, for the well ascertained fact that, of those phanerogamous plants which are common to very distant countries, (excluding those introduced thro’ cultivation or other human agencies,) the majority belong to the division of glumaceous monocotyledons (Grasses & Sedges), & most of the remainder are aquatic or marsh plants? Are there any freshwater shells or insects common to Europe & the Cape?

The only flowering plant which is at once North American & British, & at the same time not found on the continent of Europe, is a water plant, Eriocaulon septangulare; & it is one which does not seem to have any peculiar facilities for migration. How did it cross over from Canada to Connemara & the Isle of Skye? Many & puzzling are the questions of this sort which occur to one.

Again, as to the Cape:—poor E. Forbes, I remember, thought that there was an axis of high land running up through central Africa, connecting the plateau of the Cape with that of Abyssinia;⁴ & it is true that there are some remarkable botanical analogies (but I believe no case of specific identity) between those two regions.⁵ Still, this would help us but very little way towards an overland route for plants between Europe & the Cape; for between Abyssinia, & the Mediterranean there is a huge gap, the arid deserts of Nubia & the low burning valley of the Nile. There are indeed but few species common to Europe & the Cape, but those few are very difficult to account for.

I want to know whether you agree in the opinion which I have expressed in my Cape book,⁶ that the zoology of that country is more analogous to that of the rest of Africa, than its botany? I do not at all know how far it may hold with respect to the invertebrate animals, but (as far as I can depend upon my very slight knowledge of zoology,) it strikes me as holding good to a certain extent with the quadrupeds, birds, & even the serpents. I wish I could visit Natal, which is now far more accessible than when I was at the Cape. It seems clear that as far as the Flora of the Cape is at all connected with that of tropical Africa, it is so in that direction, by the east coast & the Quathlamba mountains; whereas towards the N.W. & N. it is entirely cut off by the deserts. It appears from Krauss’s account (which you will find in the Ray Soc. volume for 1849, p. 387,)⁷ that at Natal there are many tropical forms on the coast, with Aloes & tall Euphorbias on the hills, & several species of peculiar Cape genera in the mountain meadows. I am sure I have read, tho’ I cannot at this moment recollect where, that the Doum Palm of Upper Egypt & Nubia extends down to that south-eastern coast of Africa,—I think to Delagoa Bay.— That it is the heat & aridity of the deserts which stops the extension of the peculiar Cape Flora towards the north, seems certain. Prof. Piazzi Smyth⁸ told me that, in travelling towards the interior, north-westward from Cape Town, after crossing a vast extent of Karroo country, without any vegetation but a few scattered & stunted succulents, he ascended to the high table-land of the Bokkeveld, & found again Heaths, Proteas, & other characteristic forms of the Cape Flora, in striking contrast to the productions of the Karroo below. But why some families of plants, which abound in the western parts of the colony, are scantily represented in the same parallel in the eastern part, is by no means evident.

I have run on with this monstrous long prose about Cape botany, because I remember that, when I saw