Darwin, in The Expression of Emotion in Man and Animals, p. 222-223, referring to the muscles involved in furrowing the brow during a frown:
It is not surprising that the corrugators should have become much more developed in man than in the anthropoid apes; for they are brought into incessant action by him under various circumstances, and will have been strengthened and modified by the inherited effects of use.... When the eyes are closed as quickly and as forcibly as possible, to save them from being injured by a blow, the corrugators contract. With savages or other men whose heads are uncovered, the eyebrows are continually lowered and contracted to serve as a shade against a too strong light; and this is effected partly by the corrugators. This movement would have been more especially serviceable to man, as soon as his early progenitors held their heads erect.
Interesting because (a) it's one of his clearer references to use inheritance; (b) it's a clear statement of comparative evolutionary anatomy applied to behavior, and (c) it presaged Grover Krantz by 100 years.
