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 (comparatively speaking) in making a
horizontal turn of the head--can only do it by a slow bend of the whole
neck." (473/2. 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.)

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. (473/3. The
error in question occurs on page 19 of the "Descent of Man," Edition 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.) 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