I long ago thought that the time would soon come when the causes of variation would be fully discussed, and no one has done so much as you in this important subject. The recent evidence of the difference between birds of the same species in the N. and S. United States well shows the power of climate. The two sexes of some few birds are there differently modified by climate, and I have introduced this fact in the last edition of my "Descent of Man." (268/2. "Descent of Man," Edition II. (in one volume), page 423. Allen showed that many species of birds are more strongly coloured in the south of the United States, and that sometimes one sex is more affected than the other. It is this last point that bears on Weismann's remarks (loc. cit., pages 44, 45) on Pieris napi. The males of the alpine-boreal form bryoniae hardly differ from those of the German form (var. vernalis), while the females are strikingly different. Thus the character of secondary sexual differences is determined by climate.) I am, therefore, fully prepared to admit the justness of your criticism on sexual selection of lepidoptera; but considering the display of their beauty, I am not yet inclined to think that I am altogether in error.
What you say about reversion (268/3. For instance, the fact that reversion to the primary winter-form may be produced by the disturbing effect of high temperature (page 7).) being excited by various causes, agrees with what I concluded with respect to the remarkable effects of crossing two breeds: namely, that anything which disturbs the constitution leads to reversion, or, as I put the case under my hypothesis of pangenesis, gives a good chance of latent gemmules developing. Your essay, in my opinion, is an admirable one, and I thank you for the interest which it has afforded me.
P.S. I find that there are several points, which I have forgotten. Mr. Jenner Weir has not published anything more about caterpillars, but I have written to him, asking him whether he has tried any more experiments, and will keep back this letter till I receive his answer. Mr. Riley of the United States supports Mr. Weir, and you will find reference to him and other papers at page 426 of the new and much-corrected edition of my "Descent of Man." As I have a duplicate copy of Volume I. (I believe Volume II. is not yet published in german) I send it to you by this post. Mr. Belt, in his travels in Nicaragua, gives several striking cases of conspicuously coloured animals (but not caterpillars) which are distasteful to birds of prey: he is an excellent observer, and his book, "The Naturalist in Nicaragua," very interesting.
I am very much obliged for your photograph, which I am particularly glad to possess, and I send mine in return.
I see you allude to Hilgendorf's statements, which I was sorry to see disputed by some good German observer. Mr. Hyatt, an excellent palaeontologist of the United States, visited the place, and likewise assured me that Hilgendorf was quite mistaken. (268/4. See Letters 252- 7.)
I am grieved to hear that your eyesight still continues bad, but anyhow it has forced your excellent work in your last essay.
May 4th. Here is what Mr. Weir says:--
"In reply to your inquiry of Saturday, I regret that I have little to add to my two communications to the 'Entomological Society Transactions.'
"I repeated the experiments with gaudy caterpillars for years, and always with the same results: not on a single occasion did I find richly coloured, conspicuous larvae eaten by birds. It was more remarkable to observe that the birds paid not the slightest attention to gaudy caterpillars, not even when in motion,--the experiments so thoroughly satisfied my mind that I have now given up making them."
LETTER 269. TO LAWSON TAIT.
(269/1. The late Mr. Lawson Tait wrote to Mr. Darwin (June 2nd, 1875): "I am watching a lot of my mice from whom I removed the tails at birth, and I am coming to the conclusion that the essential use of the tail there is as a recording organ--that is, they record in their memories the corners they turn and the height of the holes they pass through by touching them with their tails." Mr.