the will in any animal. In such cases some explanation, quite distinct
from habit, will have to be discovered. The radiation of nerve-force
from strongly-excited nerve-cells to other connected cells, as in the
case of a bright light on the retina causing a sneeze, may perhaps aid
us in understanding how some reflex actions originated. A radiation of
nerve-force of this kind, if it caused a movement tending to lessen the
primary irritation, as in the case of the contraction of the iris
preventing too much light from falling on the retina, might afterwards
have been taken advantage of and modified for this special purpose.
It further deserves notice that reflex actions are in all probability
liable to slight variations, as are all corporeal structures and
instincts; and any variations which were beneficial and of sufficient
importance, would tend to be preserved and inherited. Thus reflex
actions, when once gained for one purpose, might afterwards be modified
independently of the will or habit, so as to serve for some distinct
purpose. Such cases would be parallel with those which, as we have
every reason to believe, have occurred with many instincts; for
although some instincts have been developed simply through
long-continued and inherited habit, other highly complex ones have been
developed through the preservation of variations of pre-existing
instincts that is, through natural selection.
I have discussed at some little length, though as I am well aware, in a
very imperfect manner, the acquirement of reflex actions, because they
are often brought into play in connection with movements expressive of
our emotions; and it was necessary to show that at least some of them
might have been first acquired through the will in order to satisfy a
desire, or to relieve a disagreeable sensation.
_Associated habitual movements in the lower animals_. I have already
given in the case of Man several instances of movements associated with
various states of the mind or body, which are now purposeless, but
which were originally of use, and are still of use under certain
circumstances. As this subject is very important for us, I will here
give a considerable number of analogous facts, with reference to
animals; although many of them are of a very trifling nature. My object
is to show that certain movements were originally performed for a
definite end, and that, under nearly the same circumstances, they are
still pertinaciously performed through habit when not of the least use.
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