th one side next to the ground, and of partially or wholly
burying themselves in the mud, always, however, with one side down. They
thus became flat fishes, losing the coloring of their under surfaces,
and their eyes migrating across their foreheads and taking up positions
on the upper surfaces of their heads. Again, when animals are placed
among surroundings in which there is no need for some special organ,
this organ degenerates, and passes wholly or partially into a
rudimentary condition, or, entirely out of existence. These latter
effects of changed conditions on animals are especially noticeable in
the effect of continual darkness on the organs of sight of those
creatures which, owing to said mutations, have 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 li