When vomiting results, as a reflex action, from some real cause--
as from too rich food, or tainted meat, or from an emetic--it does not
ensue immediately, but generally after a considerable interval of time.
Therefore, to account for retching or vomiting being so quickly and easily
excited by a mere idea, the suspicion arises that our progenitors
must formerly have had the power (like that possessed by ruminants
and some other animals) of voluntarily rejecting food which disagreed
with them, or which they thought would disagree with them; and now,
though this power has been lost, as far as the will is concerned,
it is called into involuntary action, through the force of a formerly
well-established habit, whenever the mind revolts at the idea
of having partaken of any kind of food, or at anything disgusting.
This suspicion receives support from the fact, of which I am assured
by Mr. Sutton, that the monkeys in the Zoological Gardens often vomit
whilst in perfect health, which looks as if the act were voluntary.
We can see that as man is able to communicate by language to his
children and others, the knowledge of the kinds of food to be avoided,
he would have little occasion to use the faculty of voluntary rejection;
so that this power would tend to be lost through disuse.
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