“Down. “DEAR SIR,--I am much obliged for your kindness in informing me of the hexadactylous case; but so many have been recorded that I do not think, except under very special circumstances, it would be worth your while further to investigate it. “With my thanks, yours faithfully and obliged, “CH. DARWIN.” The next letter refers to Meldola’s communication to _Nature_ (he had recently written upon pangenesis and upon sexual selection), and his work on mimicry, protective resemblance, etc. In the latter part we meet with an interesting reference to the researches on cross-fertilisation which are now so famous. “_June 9th [1871]._ “Down. “DEAR SIR,--I am greatly obliged by your note. I have read with much interest and carefully perused your letter in Nature, and am looking out for a paper announced for Linn. Soc. Your remarks shall all be in due time fully considered. With respect to the separation of the sexes, I have often reflected on the subject; but there is much difficulty, as it seems to me and as Nageli has insisted, inasmuch as a strong case can be made out in favour of the view that with plants at least the sexes were primordially distinct, then became in many cases united, and in not a few cases re-separated. I have during the last 5 or 6 years been making a most laborious series of experiments, by which I shall be able, I think, to demonstrate the wonderful good derived from crossing, and I am almost sure but shall not know till the end of the summer that I shall be able to prove that the good is precisely of the same kind which the adult individual derives from _slight_ changes of conditions. “With my sincere thanks for your interest in my work, I remain, dear Sir, Yours very faithfully, “CH. DARWIN.” The following letter is of great interest in relation to many problems of sexual selection, protective resemblance, mimicry, etc.:-- “_Jan. 23, 1872._ “Down.