Darwin was interested in the idea because "some German sneered at Natural Selection and instanced the tails of mice.")
June 11th, 1875.
It has just occurred to me to look at the "Origin of Species" (Edition VI., page 170), and it is certain that Bronn, in the appended chapter to his translation of my book into german, did advance ears and tail of various species of mice as a difficulty opposed to Natural Selection. I answered with respect to ears by alluding to Schobl's curious paper (I forget when published) (269/2. J. Schobl, "Das aussere Ohr der Mause als wichtiges Tastorgan." "Archiv. Mik. Anat." VII., 1871, page 260.) on the hairs of the ears being sensitive and provided with nerves. I presume he made fine sections: if you are accustomed to such histological work, would it not be worth while to examine hairs of tail of mice? At page 189 I quote Henslow (confirmed by Gunther) on Mus messorius (and other species?) using tail as prehensile organ.
Dr. Kane in his account of the second Grinnell Expedition says that the Esquimaux in severe weather carry a fox-tail tied to the neck, which they use as a respirator by holding the tip of the tail between their teeth. (269/3. The fact is stated in Volume II., page 24, of E.K. Kane's "Arctic Explorations: The Second Grinnell Expedition in Search of Sir John Franklin." Philadelphia, 1856.)
He says also that he found a frozen fox curled up with his nose buried in his tail.
N.B. It is just possible that the latter fact is stated by M'Clintock, not by Dr. Kane.
(269/4. The final passage is a postscript by Mr. W.E. Darwin bearing on Mr. Lawson Tait's idea of the respirator function of the fox's tail.)
LETTER 270. TO G.J. ROMANES. Down, July 12th, 1875.
I am correcting a second edition of "Variation under Domestication," and find that I must do it pretty fully. Therefore I give a short abstract of potato graft-hybrids, and I want to know whether I did not send you a reference about beet. Did you look to this, and can you tell me anything about it?
I hope with all my heart that you are getting on pretty well with your experiments.
I have been led to think a good deal on the subject, and am convinced of its high importance, though it will take years of hammering before physiologists will admit that the sexual organs only collect the generative elements.
The edition will be published in November, and then you will see all that I have collected, but I believe that you gave all the more important cases. The case of vine in "Gardeners' Chronicle," which I sent you, I think may only be a bud-variation not due to grafting. I have heard indirectly of your splendid success with nerves of medusae. We have been at Abinger Hall for a month for rest, which I much required, and I saw there the cut-leaved vine which seems splendid for graft hybridism.
LETTER 271. TO FRANCIS GALTON. Down, November 7th, 1875.
I have read your essay with much curiosity and interest, but you probably have no idea how excessively difficult it is to understand. (271/1. "A Theory of Heredity" ("Journal of the Anthropological Institute," 1875). In this paper Mr. Galton admits that the hypothesis of organic units "must lie at the foundation of the science of heredity," and proceeds to show in what respect his conception differs from the hypothesis of pangenesis. The copy of Mr. Galton's paper, which Darwin numbered in correspondence with the criticisms in his letter, is not available, and we are therefore only able to guess at some of the points referred to.) I cannot fully grasp, only here and there conjecture, what are the points on which we differ. I daresay this is chiefly due to muddy-headedness on my part, but I do not think wholly so. Your many terms, not defined, "developed germs," "fertile," and "sterile germs" (the word "germ" itself from association misleading to me) "stirp," "sept," "residue," etc., etc., quite confounded me. If I ask myself how you derive, and where you place the innumerable gemmules contained within the spermatozoa formed by a male animal during its whole life, I cannot answer myself.