140 MEMOIES OF THE NATIONAL ACADEMY OF SCIENCES. 



possessed these eye-spots; in the remaining 65 to 70 per cent, they were absent, as well as the optic nerves; while 

 there was only one, or even no, recognizable rudiment of an optic ganglion. He also found that the offspring of par- 

 ents, both of which had eyes, were themselves provided with them; but that if either the father or the mother were 

 blind, the young were also blind, having at most a feeble indication of optic lobes. Dr. Stecker considers this a most 

 instructive case of the gradual atrophy of an organ by disuse, owing to the influence of changed conditions. There 

 can be little doubt that the ancestors of Chernes possessed well-developed eyes ; the first steps in the retrogressive 

 process was the loss of the cornea and cones, the optic nerve and ganglion remaining after the true percipient appa- 

 ratus had gone.* 



Here is a fertile field for careful aud long-continued observations on animals reared in different 

 degrees of darkness. Such experiments will afford a crucial test of the theory of the comparatively- 

 rapid evolution of genera and species due to sudden changes in the environment. 



It is evident that physiological experiments are needed as well as embryological studies to 

 throw further light on the origin of cave animals. The blind-fish, blind crayfish, and Csecidotaea, 

 which might be reared in dark cellars, should be observed for a series of generations, to ascertain 

 whether by breeding the eyes can not be restored and the species by artificial means be induced 

 to revert to its ancestral type. The embryology of the cave beetles, with or without rudimentary 

 eyes, of the eyeless spiders and of Myriopods, of the Osecidotsea, and of the blind crayfish and blind- 

 fish should be carefully worked out as regards the presence of organs of vision in a rudimentary 

 state, though we should hardly expect to find rudimentary eyes in Anophthalmus when both larva 

 and pupa do not possess them. 



ISOLATION AS A FACTOR IN THE ORIG-IN OP CAYE ANIMALS. 



When any cave, such as Mammoth or Wyandotte, etc., is once colonized by emigrants from 

 the upper world, and the colonists becoming adapted to the new conditions environing them, have 

 lost their eye-sight or even all traces of eyes, and the new forms thus established begin to breed 

 true to their recently acquired characteristics, it is obvious that this process of in-and-in breeding 

 will continue as long as the new forms live in total darkness and are isolated from the allied forms 

 or their eyed ancestors of the upper world of light. Though a subordinate factor, isolation is 

 certainly of no little importance in securing the stability of the new species and genera. It is 

 evident that if uo stragglers from the upper world, as sp cies of Trechus to interbreed with the 

 cave Anophthalmi, species of Choleva to cross with Adelops or Bathyscia, or species of Ceutho- 

 philus to mix with the true cave Ceuthophili, or species of Myriopods or Arachnida to intercross 

 with the cave forms, then the latter will tend to remain as fixed as we now find them to be. In 

 the case of the crayfish of Mammoth Cave, the normal Caynbarus bartonii, introduced at times of 

 heavy rains or freshets into the cave, is not seldom found living in company with Oreoneetes pel- 

 lucidus, the blind form, but belonging to a different section of the genus as regards the shape of 

 its gonopods or first male abdominal appendages, and being of much larger size, it is probably 

 incapable of fertilizing the eggs of the blind form, even if the latter, timid and sensitive to the 

 least disturbance of the water, should allow itself to be approached by the larger-eyed form. It 

 is also probable that Ccecidotcea stygia is seldom, if ever, brought in contact with Asellus com- 

 munis, wuich abounds in the pools and streams throughout the cave region. I have never found 

 a stray Asellus even partly bleached and with dimished eyes in any caves, nor seen such specimens 

 in collections made by others, though they may yet be found. Whether living in caves or wells 

 fed by subterranean streams, the bleached, eyeless, or nearly eyeless, forms breed true to their 

 type and show no signs of intercrossing with lucophilous forms. 



Should, however, these cave forms be placed in such circumstances as to be able to mix or 

 intercross with their epigean allies, which are in all probability the very species to which they 

 owe their origin, there would with little doubt be a constant tendency to revert to the ancestral 

 eyed forms, and we should constantly find certain individuals with visual organs better de- 

 veloped, and with a darker integument, serving as connecting links. Such links may have been 

 common enough when the caves were first formed and colonized, and in some species, as Pseudo- 

 tremia cavernarum, they frequently occur at the present time, but, as a rule, owing to long 

 isolation or seclusion, and the consequent impossibility of intercrossing, they are now rare. 



*Morp. Jahrbuch, iv, 279, 1878; Journ. Roy. Micr. Soc, ii, 146, 1879. 



