or horse starts, his heart beats wildly against his ribs, and here it
may be truly said we have an organ which has never been under the control
of the will, partaking in the general reflex movements of the body.
To this point, however, I shall return in a future chapter.

The contraction of the iris, when the retina is stimulated
by a bright light, is another instance of a movement,
which it appears cannot possibly have been at first voluntarily
performed and then fixed by habit; for the iris is not known
to be under the conscious control of 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