A bird's chest is essentially a pair of bellows in which the sternum during rest and the back during flight act as movable wall. The air cells may all be represented as soft-walled bags opening freely into the bellows--there being, so far as anatomists yet know, no valves or corresponding contrivances anywhere except at the glottis, which corresponds with the nozzle and air valve both, of our bellows. But the glottis is always opened when the chest is dilated at each inspiration. How then can the air in any air-cell be kept at a higher tension than the surrounding atmosphere? Hunter experimented on the uses of the air sacs, I know, but I have not his work at hand. It may be that opening one of the air-cells interferes with flight, but I hold it very difficult to conceive that the interference can take place in the way you suppose. How on earth is a lark to sing for ten minutes together if the air-cells are to be kept distended all the while he is up in the air? At any rate twenty other illustrations will answer your purpose as well, so I would not select one which may be assailed by a carping fellow like Yours very faithfully, T.H. Huxley. October 10, 1860. My dear Spencer [This was written at the time when Mr. Spencer had issued a notice of discontinuance, and when measures were being taken to prevent it.], "A wilful man must have his way," and if you won't let me contribute towards the material guarantees for the success of your book, I must be content to add twelve shillings' worth of moral influence to that I already meant to exert per annum in its favour. I shall be most glad henceforth, as ever, to help your great undertaking in any way I can. The more I contemplate its issues the more important does it seem to me to be, and I assure you that I look upon its success as the business of all of us. So that if it were not a pleasure I should feel it a duty to "push behind" as hard as I can. Have you seen this quarter's "Westminster?" The opening article on "Neo-Christianity" is one of the most remarkable essays in its way I have ever read. I suppose it must be Newman's. The "Review" is terribly unequal, some of the other articles being absolutely ungrammatically written. What a pity it is it cannot be thoroughly organised. My wife is a little better, but she is terribly shattered. By the time you come back we shall, I hope, have reverted from our present hospital condition to our normal arrangements, but in any case we shall be glad to see you. Ever yours faithfully, T.H. Huxley. [The following is, I think, the first reference to his fastidiousness in the literary expression and artistic completeness of his work. As he said in an after-dinner speech at a meeting in aid of the Literary Fund, "Science and literature are not two things, but two sides of one thing." Anything that was to be published he subjected to repeated revision. And thus, apologising to Hooker for his absence, he writes (August 2, 1860):--]