On this one point of beauty, I can hardly think that the Duke was quite candid. I have used in the concluding paragraph of my present book precisely the same argument as you have, even bringing in the bulldog,[63] with respect to variations not having been specially ordained. Your metaphor of the river[64] is new to me, and admirable; but your other metaphor, in which you compare classification and complex machines, does not seem to me quite appropriate, though I cannot point out what seems deficient. The point which seems to me strong is that all naturalists admit that there is a _natural_ classification, and it is this which descent explains. I wish you had insisted a little more against the _North British_[65] reviewer assuming that each variation which appears is a strongly marked one; though by implication you have made this _very_, plain. Nothing in your whole article has struck me more than your view with respect to the limit of fleetness in the racehorse and other such cases; I shall try and quote you on this head in the proof of my concluding chapter. I quite missed this explanation, though in the case of wheat I hit upon something analogous. I am glad you praise the Duke's book, for I was much struck with it. The part about flight seemed to me at first very good, but as the wing is articulated by a ball-and-socket joint, I suspect the Duke would find it very difficult to give any reason against the belief that the wing strikes the air more or less obliquely. I have been very glad to see your article and the drawing of the butterfly in _Science Gossip_. By the way, I cannot but think that you push protection too far in some cases, as with the stripes on the tiger. I have also this morning read an excellent abstract in the _Gardeners' Chronicle_ of your paper on nests;[66] I was not by any means fully converted by your letter, but I think now I am so; and I hope it will be published somewhere _in extenso_. It strikes me as a capital generalisation, and appears to me even more original than it did at first. I have had an excellent and cautious letter from Mr. Geach of Singapore with some valuable answers on expression, which I owe to you. I heartily congratulate you on the birth of "Herbert Spencer," and may he deserve his name, but I hope he will copy his father's style and not his namesake's. Pray observe, though I fear I am a month too late, when tears are first secreted enough to overflow; and write down date. I have finished Vol. I. of my book, and I hope the whole will be out by the end of November; if you have the patience to read it through, which is very doubtful, you will find, I think, a large accumulation of facts which will be of service to you in your future papers, and they could not be put to better use, for you certainly are a master in the noble art of reasoning. Have you changed your house to Westbourne Grove? Believe me, my dear Wallace, yours very sincerely, CH. DARWIN.