392 THE POPULAR SCIENCE MONTHLY. 



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 rudi- 

 mentary conditions of the eye. The atrophy is comparatively 

 rapid, sudden, and wholesale. On the other hand, we have series, 

 as in Ccecidoicea or Chtho?iius, where there is but a single, or two 

 or three, or several crystalline lenses, partially enveloped in pig- 

 ment. 



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. For example, those individuals of Chthonius Pack- 

 ardii which live in the Labyrinth 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 the Cceci- 

 dotcea are eyeless, a few have rudimentary eyes. Thus, in the dif- 

 fering conditions of the eyes in different individuals, we have an 

 epitome of the developmental history of the genus Ccecidotcea and 

 its species. Certain Aselli borne into caves or introduced into 

 subterranean 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 correspond- 

 ing lenses and retina ; 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 retina, and those complicated, elaborate structures, 

 the optic ganglion and optic nerve, 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 inheritance 

 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 all the cave ani- 

 mals 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 Ccecidotcea would 

 perhaps be the best subject for such an experiment ; it is so abun- 

 dant and easy to breed. That the Ccecidotcea has been evolved 



