i86S 1874] EXPRESSION 



any part to which the mind maybe intently and long directed. Letter 472 

 For instance, if thinking intently about a local eruption on the 

 skin (not on the face, for shame might possibly intervene) 

 caused it temporarily to redden, or thinking of a tumour 

 caused it to throb, independently of increased heart action. 



To Hubert Airy. Letter 473 



Dr. Airy had written to Mr. Darwin on April 3rd : 

 " With regard to the loss of voluntary movement of the ears in man 

 and monkey, may I ask if you do not think it might have been caused, as 

 it is certainly compensated, by the facility and quickness in turning the 

 head, possessed by them in virtue of their more erect stature, and the 

 freedom of the atlanto-axial articulation ? (in birds the same end is 

 gained by the length and flexibility of the neck.) The importance, in 

 case of danger, of bringing the eyes to help the ears would call for a 

 quick turn of the head whenever a new sound was heard, and so would 

 tend to make superfluous any special means of moving the ears, except 

 in the case of quadrupeds and the like, that have great trouble (com- 

 paratively speaking) in making a horizontal turn of the head can only 

 do it by a slow bend of the whole neck." l 



Down, April 5th [1871]. 



I am greatly obliged for your letter. Your idea about 

 the easy turning of the head instead of the ears themselves 

 strikes me as very good, and quite new to me, and I will 

 keep it in mind ; but I fear that there are some cases opposed 

 to the notion. 



If I remember right the hedgehog has very human ears, 

 but birds support your view, though lizards are opposed to it. 



Several persons have pointed out my error about the 

 platysma. 2 Nor can I remember how I was misled. I find 

 I can act on this muscle myself, now that I know the corners 

 of the mouth have to be drawn back. I know of the case 

 of a man who can act on this muscle on one side, but not on 

 the other ; yet he asserts positively that both contract when 

 he is startled. And this leads me to ask you to be so kind 

 as to observe, if any opportunity should occur, whether the 



1 We are indebted to Dr. Airy for furnishing us with a copy of his 

 letter to Mr. Darwin, the original of which had been mislaid. 



2 The error in question occurs on p. 19 of the Descent of Man, Ed. I., 

 where it is stated that the Platysma myoides cannot be voluntarily brought 

 into action. In the Expression of the Emotions Darwin remarks that 

 this muscle is sometimes said not to be under voluntary control, and 

 he shows that this is not universally true. 



