been compelled to dwell in darkness for
untold ages.
The mole, far back in the past, had eyes, and gained its livelihood
above ground in the broad light of day; but, owing to some change in
its surroundings, it was forced to burrow beneath the surface of the
earth; consequently its organs of sight have degenerated, and are now
practically worthless as far as vision is concerned. All moles, however,
can tell darkness from light, consequently, are not wholly blind--a
certain amount of sight remains. This is due to the fact that, although
the optic nerve, on examination, is invariably found to be atrophied or
wasted, there yet remain in the shrivelled nerve-cord true nerve-cells;
these nerve-cells transmit light impressions to the brain.
Even if the optic nerves, and, in fact, all of the structures of the eye,
were absent, I yet believe that the mole could differentiate between
daylight and darkness. The sensitive tufts and filaments of nerve in the
skin, undoubtedly, in many instances, respond to the stimulation of
light, so that totally blind animals, animals with no rudimentary organs
of vision whatever, and the inception of whose ancestors, themselves
wholly blind, probably took place thousands of years ago, show by
their actions that light is exceedingly unpleasant to them. Thus, I have
seen actinophryans taken from the River Styx in Mammoth Cave
(which is their natural habitat), seeking to hide themselves beneath a
grain of sand which happened to be drawn up in the pipette and
dropped upon the glass slide beneath the object-glass of my
microscope.
I have repeatedly seen the blind fish of Mammoth Cave seeking out the
darkest spots in aquaria. In point of fact, I think it can be demonstrated
that light is directly fatal to these fishes; they soon die when taken from
the river and placed in aquaria where there is an abundance of light.
These fish, although they have rudimentary eyes, never have the
slightest remaining trace of nerve-cells in the wasted optic nerve (that is,
I have never been able to discover any), thus showing that their
appreciation of light is not derived through the agency of their eyes. An
eyeless spider (Anthrobia) taken from the same cavern showed a like
distaste for light, and yet, in this insect, there is absolutely no vestige of
an eye or its nerves.
Finally, a friend of mine, a youth of eighteen, totally blind since birth,
can differentiate between daylight and darkness. On one occasion I
carefully blindfolded him and led him into the well-lighted office of a
brewery (he had never been in a brewery before), and asked him if it
were light or dark. He answered that it was almost as light as day. I
then conducted him into the dark beer vaults, and as soon as he passed
the door he exclaimed, "How cold and dark it is here!" Thinking that he
might possibly associate darkness with coldness, I asked him if this
were the case. "No," he replied, "I see the darkness and I feel the cold;
they are not the same."
In these animals--and I include man--continuous darkness has modified
sensibility (sense of touch) to such an extent that it has partially taken
on the functions of the useless organs--the eyes; these creatures see
with their skins.
I do not believe that there is a creature in existence to-day, whether it
has eyes or not, which cannot tell the difference between night and day.
Professor Semper says that in the Pelew Islands he found a small
fresh-water creature, whose generic name is Cymothoe, in pools where
daylight penetrated, that was absolutely blind.[6] We have fresh-water
Cymothoe living in our own waters that are close kin to the Pelew
islander mentioned by Semper, and which are not blind. Along the
middle of their backs, over the edge of each segment, there is an oblong
dark spot. This little collection of coloring-matter is covered by a
transparent membrane, the cornea, and has a special nerve leading to
the brain, if I may use the word. These spots are primitive eyes, the
analogues of which are preserved by many of the true worms. I am
inclined to believe that Semper would find primitive eyes of some form
or other in the Cymothoe he mentions, if he were again to examine 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
Continue reading on your phone by scaning this QR Code
Tip: The current page has been bookmarked automatically. If you wish to continue reading later, just open the
Dertz Homepage, and click on the 'continue reading' link at the bottom of the page.