But such experience will have been slowly gained at a later age during
a long series of generations; and from what we know of inheritance,
there is nothing improbable in the transmission of a habit to the offspring
at an earlier age than that at which it was first acquired by the parents.
From the foregoing remarks it seems probable that some actions,
which were at first performed consciously, have become
through habit and association converted into reflex actions,
and are now so firmly fixed and inherited, that they
are performed, even when not of the least use,[14] as often
as the same causes arise, which originally excited them in us
through the volition. In such cases the sensory nerve-cells
excite the motor cells, without first communicating with
those cells on which our consciousness and volition depend.
It is probable that sneezing and coughing were originally
acquired by the habit of expelling, as violently as possible,
any irritating particle from the sensitive air-passages. As far
as time is concerned, there has been more than enough for these
habits to have become innate or converted into reflex actions;
for they are common to most or all of the higher quadrupeds,
and must therefore have been first acquired at a very remote period.
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