My dear Bunbury

I am very much obliged to you for writing to me at such length; for I value your opinion much. Ever since our sojourn at Capel Kurig² I have formed (if you will not think it impertinent in me to say so) the highest opinion of your knowledge, acuteness & judgment. Therefore I thank you much for your praise of my Book. I am sorry, but not in the least surprised that you do not go a little further with me.³ I changed so slowly myself that I am indeed surprised at anyone becoming a convert.—

I will just run through some of the salient points in your letter: with respect to admitting representative species to be descendants of one species & not extending the same view to whole classes, the point seems to me to turn whether you think the explanation which I offer of the classification of all Beings, the principles of Homology & of Embryology, & Rudimentary organs are satisfactorily explained.⁴ The explanation seems to me to be nearly satisfactory; & this being so, I can to myself allow of no difficulties in imagining what were the steps of transition to weigh against the understanding such large classes of facts.— It is curious how differently different men view the same subject; Asa Gray thinks the Hybrid Ch^r. one of strongest in Book!⁵ But he says (& I quite agree) more wants making out why varieties are not sterile when crossed. But this seems hopeless till we can say on what difference two close species are to certain extent fertile, & another two close species absolutely sterile.

With respect to Nat. Selection not being a “vera causa”; it seems to me fair in Philosophy to invent any hypothesis & if it explains many phenomena it comes in time to be admitted as real.⁶ In your sense the undulatory theory of the hypothetical ether (the undulations themselves being not recognised) is not a vera causa in accounting for all the phenomena of Light. Natural selection seems to me in so far in itself not be quite hypothetical, in as much if there be variability & a struggle for life, I cannot see how it can fail to come into play to some extent. You speak of “Degeneracy”,⁷ I wish you could give me any instance of a race, kept under the same conditions. & in a moderately large body together degenerating or modifying: this is always assumed as certain; I do not dispute it, but I wish much for some evidence.

The extinction of intermediate forms seems to me explained in Ch. IV & more especially in the X^th Chapt.: if my Principle of Divergence of Character be admitted this almost necessarily follows.

I quite agree with what you say on effect of admission of theory on Systematic work; (see p. 485), not but what I was haunted with endeavouring to guess what cirripedes would be ranked as species by other naturalists.⁸ I think the importance of theory bears on opening up new fields of enquiry & in giving a rational, instead of theological explanation of many known facts   I am much pleased to hear that you intend reading the Book again. If not too troublesome I shall be most grateful for any other remarks. Your letter is as clear as daylight, & mine, I fear, like a London foggy day.—

Again I thank you heartily for all your kind trouble & believe me | Dear Bunbury | Yours very sincerely | Charles Darwin

As you give up representative species, I suspect & hope that you will go further; but this will depend wholly whether you think the large classes of facts in Ch. XIII are explained by Theory.— See remark (at p 127) where I say “Whether nat. Select. has really thus acted in nature in modifying &c &c must be judged of by the balance of evidence given in the following Chapters”