mine it.
The insects, etc., which dwell in caves, and which have eyes, are new
arrivals; they have not dwelt long enough in total darkness to have
experienced the full effects of changed surroundings. They show,
however, that they are beginning to feel such effects, for there is more
or less diminution in the color-cells of the eyes and body coverings. My
experiments on fish and frogs show, conclusively, that the
color-producing function is directly due to light stimulation. The
longer fish and frogs are kept in total darkness, the lower is the
number of color-cells and the smaller is the amount of coloring-matter.
This accounts for the fact that all animals which have dwelt in darkness
for untold ages are absolutely colorless. Pigmented or colored fishes,
nevertheless, having well-developed organs of vision, have been taken
from such depths (over a mile) as to preclude the possibility of a
single ray of daylight.[7] These fishes, however, are phosphorescent,
and thus furnish their own light. Moreover, I am inclined to believe
that the vast depths of the ocean, in certain localities, lie bathed in
a continuous phosphorescent glow, so that creatures living there neither
lose their color nor their eyes, sufficient light being present to
prevent degeneration. Where eyeless and colorless fishes are brought up
from great depths, there the ocean is not phosphorescent, but is in
absolute darkness.

[6] Semper, _Animal Life_, p. 83.

[7] Hickson, _The Fauna of the Deep Sea_, p. 150 _et seq._

The preceding observations indicate that the sense of sight is a very
old sense, and that it is to be found in a primitive form (ocelli) in
animals of exceedingly low organization. That this is true, I will now
attempt to demonstrate.

Sight is the result of the conversion of one form of motion into
another--a conservation, as it were, of energy. Thus, waves of light
coming from a luminous body are arrested by the pigment-cells of the
retina in our eyes and are transmuted into another form of motion, which
is called nerve energy (in this instance, sight). It would seem that as
far as sight (_vision_ is not included) is concerned, eyes of very
simple construction would amply satisfy the needs of thousands of
creatures whose existence does not depend upon vision. This supposition
is undoubtedly correct; there are many creatures in existence to-day
with eyes so exceedingly simple that they can form no visual picture of
objects--they