From W. G. Walker   6 December 1874

Dear Sir,

Last year I took the liberty on addressing you on a point that occurred to me in connection with your “Expression of the Emotions”.¹ I have just finished reading the work a second time, & venture again to write to you. At the same time, please state unreservedly whether communications (of which you doubtless receive a great many) from outsiders are a nuisance to you.

You say (p. 259) “the suspicion arises that our progenitors must formerly have had the power (like that possessed by ruminants & some other animals) of voluntarily rejecting food which disagreed with them or which they thought would disagree with them”. The explanation is certainly plausible, & perhaps probable, but the conduct of the dog seems to me to present a difficulty. I suppose it is a well-known fact (I have myself observed it several times) that dogs vomit when in a perfect state of health. If this were all, it might well be supposed that they vomited intentionally, to relieve their stomachs of what is, or what they suppose to be, an unwholesome load. But when they have rejected food in this manner, they shortly afterwards proceed to devour what they have so rejected,—‘returning like a dog to his vomit’. If the food is, or is supposed to be, injurious,—whether the vomiting be voluntary or involuntary,—what can be their motive for taking it again into their stomachs?²

Trusting I shall not be thought impertinent for proposing this point, | I remain, | Faithfully yours, | W. Gregory Walker.

Ch. Darwin, Esq^r.