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118 MEMOIRS OP THE NATIONAL ACADEMY OP SCIENCES. 
1. Total atrophy of optic lobes and optic nerves, with or without the persistence in part of 
the pigment or retina and the crystalline lens (Caecidotsea, Crangonyx, Chthonius, Adelops, 
Pseudotremia). 
2. Persistence of the optic lobes and optic nerves, but total atrophy of the rods and cones, 
retina (pigment) and facets (Orconectes). 
3. Total atrophy of the optic lobes, optic nerves, and all the optic elements, including rods 
and cones, retina (pigment) and facets (Anophthalmus, Scoterpes, and ? Anthrobia). 
An interesting fact confirmatory of the theory of occasional rapid evolution, as opposed to 
invariably slow action involved in pure Darwinism, is that we never find any rudiments of the optic 
lobes or optic nerves; if they are wanting at all, they are totally abolished; there is not a series of 
individuals with these organs in different degrees of development corresponding to the rudimen¬ 
tary conditions of the eye. The atrophy is comparatively rapid, sudden, and wholesale. On the 
other hand, we have series, as in Csecidotsea or Chthonius, where there is but a single or two or 
three, or several crystalline lenses, partially enveloped in pigment. 
These varying degrees of development in the peripheral parts of the eye prove that the 
animals entered the caves at different periods and have been exposed for different lengths of time 
to the loss of light. Eor example, those individuals of Chthonius packardii which live in the Laby¬ 
rinth of Mammoth Cave are eyeless or have merely pigment spots; those collected in the Rotunda 
(which is much nearer the entrance to the cave) have eyes, or at least lenses and a retina. While 
most individuals of Csecidotsea are eyeless, a few have rudimentary eyes. Thus in the differing 
conditions of the eyes in different individuals we have an epitome of the developmental history 
of the genus Csecidotma and its species. Certain Aselli borne into caves or introduced into sub¬ 
terranean streams feeding deep, dark wells, losing the stimulus of the light, begin to lose their 
eyes and the power of sight. The first step is the decrease in the number of facets and corre¬ 
sponding lenses and retinae; after a few generations—perhaps in four or five—the facets become 
reduced to only four or five; the eye is then useless; then all at once, perhaps after only two or 
three generations, as a result of disuse, there is a failure in forming images on the retiua, and those 
complicated, elaborate structures, the optic ganglia and optic nerves, suddenly break down and 
are absorbed, though the external eye still exists in a rudimentary state. These imperfect lenses 
and retinae, like all rudimentary organs throughout the animal world, are like ancient, decayed 
sign-posts, pointing out some nearly obliterated path now unworn and disused. The result of 
change of environment, with disuse and atrophy of the organs of vision,'together with the inher¬ 
itance of these defects and their establishment as fixed specific and even generic characters, 
results in the creation of a new natural genus with its assemblage of species, and, if we include 
ail the cave animals thus produced, the creation of a new fauna. It would be a thorough test of 
the theory of descent if we could keep these creatures in confinement exposed first to twilight 
and then to the full light of day, and endeavor to breed a few generations of these blind animals 
and ascertain whether their descendants would not revert to the original ancestral eyed forms. 
The Csecidotaea would perhaps be the best subject for such an experiment; it is so abundant 
and easy to breed. That the Csecidotaea has been evolved from some species of Asellus hardly 
admits of a doubt. Our Asellus communis abounds under sticks and stones, submerged boards 
and logs throughout the Northern and Central States. From thence it could readily be carried, 
in cavernous regions like those of southern Illinois, Indiana, and Kentucky, into subterranean 
streams. The supply must be very great, as the individuals of C. stygia are very abundant; 
indeed, so far as we know, as much or even more so than those of Asellus communis. 
In the blind crayfish of the caverns of Indiana and Kentucky, and of the similar species ( 0. 
hamulatus) inhabiting the Nickajack Cave of Tennessee, we have two aberrant forms belonging to 
a widely diffused group, whose center of distribution lies in the Mississippi Valley, and which is 
rich in species and in individuals. All the streams and ditches situated over or near the caves 
are densely populated with crayfish. I was interested, after finding C. pellucidus in a stream flow¬ 
ing through the Bradford Cave, near New Albany, Indiana, to find the common eyed crayfish of 
that region in great abundance a few yards from the mouth, outside of the cave, in the shallow 
brook issuing from the cave itself. That crayfish with eyes can readily enter a cave—probably in 
time of freshets — is proved by the fact that Cambarus bartonii is often found in Mammoth Cave; 
