IN NATURE, vol. x. p. 147, I observe a letter signed “J Guthrie,” and dated from the Cape, on the subject of the Flight of Birds, and particularly on the “hovering” of birds. It appears that one of your correspondents had referred to my chapter on this subject in the “Reign of Law” as giving a satisfactory explanation of this phenomenon. Mr. Guthrie thinks, on the contrary, that what I have there said “requires no refutation;” which is not wonderful considering the entire misconception which he evinces of the explanation I have given. He quotes me as affirming that “by a proper arrangement of its wings and tail and the position of its body, a bird can, without muscular exertion remain suspended in a horizontal air-current, provided the latter be of sufficient velocity.” If I had said this I should have talked nonsense. But I have not said it, as your readers may see by referring to the page (170, first edition) to which Mr. Guthrie himself refers. What I have said is, that under certain conditions of strength of aircurrent a kestrel can maintain the hovering position “with no visible muscular motion whatever.” Mr. Guthrie omits the word “visible,” and probably has no idea of its force and meaning in the sentence referred to. The maintenance of the wings and tail in the proper position, and of the body at the proper angle, does in itself, of course, involve continuous and difficult muscular action, although it is not visible, just as a rope-dancer standing still in some tiptoe attitude may require immense muscular effort although no motion be visible, and although the whole aim, object, and effect of that exertion be to produce stillness, and not motion.