My dear M^r Darwin

I really am very much obliged to you for having given your time to the observations on my address which I received this morning¹—which are just what I hoped to obtain from you and which will give me much to reconsider as to some of my views.

I do not wish to be supposed to underrate the importance of isolation but all I meant is that I do not think with M. Wagner that it is absolutely indispensable for the formation and maintenance of races which may in time become species²—a process evidently influenced by a great variety of causes many of which it is impossible to appreciate ⁠⟨⁠al⁠⟩⁠thou⁠⟨⁠gh⁠⟩⁠ much more study and data ⁠⟨⁠    ⁠⟩⁠ ⁠⟨⁠have⁠⟩⁠ at command—but the study of which will I trust be much facilitated by the publication of the second portion of your great work.³

I cannot think that an arid and dry climate has much effect in the concentration of a large number of species in a small space as at the Cape and S.W. Australia without the very varied protection to individuals given by a broken surface.⁴ A large proportion of the desert flora of Australia extends in identical species from the Murray and Darling northward to the Brigalow scrub of Queensland, to the N.W. Coast and to the Murchison and Sharks Bay on the W. Coast⁵ and as far as we now know is not in any part so rich as the true S.W. flora which does not extend much to the eastward of King Georges Sound nor much to the north of Avon River—⁶ but if I live to complete the Australian flora,⁷ with the additi⁠⟨⁠onal⁠⟩⁠ materials that are pouring in ⁠⟨⁠    ⁠⟩⁠ ⁠⟨⁠    ⁠⟩⁠ the interior and N. West I hope we may be able to obtain some more accurate generalisations as to that most interesting vegetation.

The flora of the arid regions of N.E Africa and Arabia is I think a widely spread and not a particularly rich one.

One effect upon species of a hot dry arid climate must be the result of propagation under those circumstances being chiefly by seed, and long preservation of individuals by underground thick rhizomes or woody bases of stems without division whilst in wet climates propagation by division—rooting of stems, suckers and runners etc assures larger proportions renovating and multiplying individuals almost without changing them—but still I do not think that extensive uniform districts have a more varied Flora when accompanied by a dry than by a moist climate. The ⁠⟨⁠    ⁠⟩⁠ in our herbaria from which any tolerably well conclusions or ⁠⟨⁠    ⁠⟩⁠ ⁠⟨⁠    ⁠⟩⁠ can be drawn are ⁠⟨⁠    ⁠⟩⁠ ⁠⟨⁠    ⁠⟩⁠ worked up.⁸

With regard to Restiaceæ Proteaceæ etc. I did not intend to intimate that I thought they once extended over the world leaving fragments in the South but that those of S. Africa and Australia must have had a common origin of very great antiquity with a sufficiently varied flora to have produced at least as many genera as there are now tribes or Orders common to the two but that the isolation has been so long and so great as to have left no species common to both countries and comparatively few groups of a higher order—⁹ What was the extent of the area of the original parents I have not presumed to conjecture. These original parents and the land they inhabited may all have perished, leaving descendants in two different countries which have diverged in different directions to the total extinction of the parent form in both countries. There are however a few plants common to Australia and South Africa which have diverged in different directions in the two countries apparently with the preservation of the common type in both—such as Pelargonium grossularioides or australe which has innumerable varieties in both countries the Australian ones varying in one set of characters the African in another,¹⁰ but one form (and that a rare one) identical in the two. Such plants as these I take to be of less antical¹¹ transportation from the one to the other country or from a common source— probably from Africa to Australia as Pelargonium is an African not an Australian type. We have no evidence of Restiaceæ Proteaceæ or any Australian African type properly so called having found its way into the northern hemisphere although Australian ones not in Africa or African ones not in Australia may have done so in very ancient times.

I had worked up some instances of evidences of common origin in widely dispersed disjointed species or rather groups of species apropos of Cassia in a paper I read last winter at the Linnean Society but which I deferred printing till I had verified some of De Candolle’s species which I did this summer—¹² and now there are other Transaction papers printing which I suppose will put off mine till the spring

With many excuses for taking up your time with my