1860 LETTERS TO SPENCER 309 



possible 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 walls. 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 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 

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

 assailed by a carping fellow like Yours very faithfully, 



T. H. HUXLEY. 



Oct. 10, 1860. 



MY DEAR SPENCER * " 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 



1 Mr. Spencer had insisted that he should be on the free list, 

 instead of being a paying subscriber to the serial issue of the 

 Synthetic Philosophy. 



