My Dear Sir.

I have to express my warmest thanks for your very kind and instructive letter of April the 30^th. as well as for the valuable present of the last edition of your admirable work “Origin of Species.”¹   it arrived last night, but I have not yet had time to peruse the chapters IX & X which I will D.V. do in a day or two being extremely pressed for time with work for the Pal. Soc. & my Indian paper for the Geol. Soc.—²

I can assure you that I shall give every attention to the subject of descent with modification, as suggested in your to me most interesting letters. I have not time alas to day to enter into details concerning the Brachiopoda, but shall not lose sight of the object & must be content at this moment to send you 3 little rough tables of the known distribution of the Brachiopoda—and which I consider to be as near the truth as the present state of our information will permit³   It is certain and no one can object to your statement relative to the great imperfection of Geological record (I was reading this morning some ob^s. upon the subject in the last number of the Geologist, for May 1861 just appeared).⁴ The Geological record is therefore very imperfect and the Palæontological record still more so—& I admit that you have a strong hold there for your theory.

Having myself on account of my monographs been obliged to examine every layer (if I may be permitted the expression) in search of Brachiopoda and to trace as far as possible the first appearance, duration, modification & extinction of each form, have been constantly (not now & then but almost every day) at a loss or puzzled what to do with intermediate shapes. did they belong to one or to the next or most closely allied species was the constant question. & more the material increased greater was the difficulty. Such work is sufficient to convince even the most incredulous of the variability of species & how a same species may become considerably modified by time and peculiar conditions— I have not the slightest doubt that by far the greater number of species of Brachiopoda could be even now linked together   The genera, perhaps also and the more our knowledge will extend the more closely will the whole become linked together   ‘Natura non facit saltum’ was my belief in 1848 & was so stated by me when I described the British Silurian Species in the Bulletin de la Soc. geol de France’⁵ and I believe that one might (tabulate the species as you propose) show a gradation & radiation, between many of the most extreme conditions.

We are very little acquainted with the rocks of the Cambrian period, but very few species of fossils have been found and that is our oldest record at present known— well in all we find orthis and Lingula only for the present, but any day might produce a larger number and especially so as you remark in the Lower Silurian the introduction of so many new genera or species. which we cannot believe to have been created for the first time then. Many of these may have been derived by modification from older strata (or from a period) still far more ancient than the upper Cambrian. When I conclude my work on the Brachiopoda I shall D.V. give a number of curious facts showing discent with modification, that many forms considered as distinct creation are only modifications.⁶ Still there are many difficulties in the way of fully realising the theory of a single creation, and I can well understand the trouble & time you have required in acquiring your conviction. I believe that the facts in Morphology & in Embryology will when more fully worked out, will cast considerable light upon many points at present but little known with reference to species.—

I am obliged to conclude this letter written in great haste, but cannot do so without once more thanking most sincerely for your very valuable communications as well as for the present of the new edition of your remarkable work.

Believe me Dear Sir | Yours very sincerely | Tho Davidson—

P.S. I forgot to say that I have not read Hookers “Introduction to the flora of Australia”,⁷ but when I next go to London will endeavour to get the loan of it from the Royal Soc. lib^y.— I have not published yet any thing upon the subject of your theory as I am still continually thinking over the matter, intending to do so at the conclusion of my work for the Pal Soc. I merely said two or three words upon the subject in a little paper published in the Geologist of which I forward a copy.⁸

to C. Darwin Esq. FRS.