For myself I am absorbed in dogs—­gone to the dogs in fact—­having been occupied in dissecting them for the last fortnight.  You do not say how your health is.

Ever yours faithfully,

T.H.  Huxley.

September 19, 1860.

My dear Spencer,

You will forgive the delay which has occurred in forwarding your proof when I tell you that we have lost our poor little son, our pet and hope.  You who knew him well, and know how his mother’s heart and mine were wrapped up in him, will understand how great is our affliction.  He was attacked with a bad form of scarlet fever on Thursday night, and on Saturday night effusion on the brain set in suddenly and carried him off in a couple of hours.  Jessie was taken ill on Friday, but has had the disease quite lightly, and is doing well.  The baby has escaped.  So end many hopes and plans—­sadly enough, and yet not altogether bitterly.  For as the little fellow was our greatest joy so is the recollection of him an enduring consolation.  It is a heavy payment, but I would buy the four years of him again at the same price.  My wife bears up bravely.

I have read your proofs at intervals, and you must not suppose 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 page 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 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