Eat an orange-pip, and you will promptly observe how effectual
is this arrangement. As a rule, the outer rind of nuts is bitter, and
the inner kernel of edible fruits. The tongue thus warns us immediately
against bitter things, as being poisonous, and prevents us automatically
from swallowing them.
'But how is it,' asks our objector again, 'that so many poisons are
tasteless, or even, like sugar of lead, pleasant to the palate?' The
answer is (you see, we knock him down again, as usual) because these
poisons are themselves for the most part artificial products; they do
not occur in a state of nature, at least in man's ordinary surroundings.
Almost every poisonous thing that we are really liable to meet with in
the wild state we are warned against at once by the sense of taste; but
of course it would be absurd to suppose that natural selection could
have produced a mode of warning us against poisons which have never
before occurred in human experience. One might just as well expect that
it should have rendered us dynamite-proof, or have given us a skin like
the hide of a rhinoceros to protect us against the future contingency of
the invention of rifles.
Sweets and bitters are really almost the only tastes proper, almost the
only ones discriminated by this central and truly gustatory region of
the tongue and palate. Most so-called flavourings will be found on
strict examination to be nothing more than mixtures with these of
certain smells, or else of pungent, salty, or alkaline matters,
distinguished as such by the tip of the tongue.
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