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From time to time letters passed between Charles Darwin and Prof. Stokes on philosophical points or on scientific business. The following letters will serve as illustrations, that from Prof. Stokes having been kindly supplied by Mr F. Darwin.

6, QUEEN ANNE ST, LONDON, W.

Thursday*.

MY DEAR PROF. STOKES,

Absence from home has prevented me from sooner thanking you most sincerely for the trouble which you have so kindly taken for me. I was rather crazy with curiosity to know what the chances were. I believe your way of stating the problem is rather better for me. I think I understand your two letters. The second way of calculating the case is much the best for me. I have made a copy for myself of your MS. sentence and have altered the few words and figures which are necessary. I cannot suppose that I have made any blunder; so if I do NOT receive your sentence back, I shall understand that it is right.

With my sincere thanks, pray believe me,

Yours truly obliged, CH. DARWIN.

DOWN, BROMLEY, KENT, S.E.

Feb. 18th, (before 1870).

MY DEAR SIR,

I have ventured to send my son to you to obtain a little information for me, on one point, if in your power to give it, and by this means you will be saved the trouble of answering this note. Have you ever attended to feathers, and can you tell me

* Mr F. Darwin points out that this letter, which he regards as characteristic, must be dated before 1868, as it refers to a passage in Variation of Animals: and Plants, Ed. i. 1868, Vol. II. p. 5. Prof. Stokes had supplied a calculation as to the chance of transmission of abnormalities in man.

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whether the splendid colours of the eye of a Peacock's tail depends on colouring matter, or on reflection? If on the latter, as appears the case, I much want to know, whether any change of structure— as the distance of a film, or the distance of fine lines or points from each other— gradually, but perhaps not equally, increasing or diminishing, would account for the series of colours, which surrounds the eye, and passes into the general tint of the barbs at the circumference of the feather. Will you be so kind as to look at the feather, and tell my son anything you can? Pray forgive me for troubling you and believe me,

My dear Sir, yours sincerely, CH. DARWIN.

Feb. 28th. [1868]

I am very much obliged to you for your great kindness in writing to me at such length about the colours of the peacock's feathers. As you say that you will look at it again, will you have the kindness to attend to one point, namely, whether a gradual thickening or thinning by little steps from the centre to the circumference, of the film of colouring matter wd account for the zones of colour which occur; or must there be zones of different kinds of colouring matter?

With very sincere thanks, believe me, my dear Sir, yours truly, CHARLES DARWIN.