230 LIFE OF PROFESSOR HUXLEY chap, xvi 



pose they have troubled me. On the contrary they were at times 

 the only things I could attend to. I agree in the spirit of the 

 whole perfectly. On some matters of detail I had doubts which 

 I am not at present clear-headed enough to think out. 



The only thing I object to in toto is the illustration which I 

 have marked at p. 24. It is physically impossible that a bird's 

 air-cells should be distended with air during flight, unless the 

 structure of the parts is in reality different from anything which 

 anatomists at present know. Blowing into the trachea is not to 

 the point. A bird cannot blow into its own trachea, and it has 

 no mechanism for performing a corresponding action. 



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 sur- 

 rounding atmosphere? 



Hunter experimented on the uses of the air sacs, I know, 

 but I have not his works 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 pur- 

 pose as well, so I would not select one which may be assailed 

 by a carping fellow like — Yours very faithfully, 



T. H. Huxley. 



Oct. ID, i860. 



My dear Spencer * — " A wilful man must have his way," 

 and if you won't let me contribute towards the material guar- 

 antees 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 



* This was written at the time when Mr. Spencer had issued a 

 notice of discontinuance, and when measures were being taken to pre- 

 vent it. 



