MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 139 



doctrine of the derivation of cave animals. Of the facts we have ourselves observed or which have 

 been observed by others we will briefly summarize : 



1. The variations in Pseudotremia cavernarum and Tomocerus plumbeus, found living near the 

 entrance of caves in partial daylight. 



13. The bleaching of Polydesmus and Machilis found living in small caves ; the blindness of 

 Neotoma, or the wood -rat of Mammoth Cave; of fish found in wells and subterranean streams; 

 the atrophy of the mole's optic nerves induced in one generation. 



3. The larger size of the eyes of the young than in the adult Troglocaris of Europe and the 

 blind crayfish of American caves ; Semper's history of the atrophy of eyes in the parasitic Pin 

 notheres; eyes of Gammarus pulex affected after living in darkness; the eyes of Gainmaridse in 

 Lake Baikel becoming smaller the deeper they live; the instability in the eyes of Caecidotaea. 



Since this paper was written a few additional instances have come to our knowledge, and 

 others are reported in periodicals bearing on this subject. While the eyeless and abyssal forms 

 (both fresh-water and deep-sea) may have existed for many generations, for periods of hundreds 

 and possibly thousands of years, yet the following facts tend to show that the bleaching of the 

 body and atrophy of the eyes, as well as the adaptations to a life in darkness, may have been 

 induced after but a few generations, perhaps but one or two only, resulting in the comparatively 

 rapid evolution of cave species. 



Thus, in a small cave near White's Cave, and at a point about 60 feet from the mouth, occurred 

 a salamander (Spelerpes longicaudatus Green) which was apparently bleached, being nearly white, 

 with dark-brown blotches. The common Cambarus bartonii occurs somewhat bleached in Mam- 

 moth Cave, and this may not be the result of inheritance, but occurs in young hatched without 

 the cave and afterwards carried in, so as not to be exposed to the light, the shell remaining pale, 

 as in the very young. Perfectly white, bleached specimens of the common Polydesmus granulatus 

 Say occurred in Indian Cave. The pale variety of Tomocerus plumbeus is possibly the product of 

 a single or at least very few generations : the white and blind Porcellio found by Mr. Hubbard in 

 Little Wyandotte Cave, though possibly a true cave form, has not yet been found elsewhere, and 

 may have been the young of a normal epigean species. But the most striking instance is the 

 bleached specimen of Asellus communis from Lost Eiver, referred to on pp. 15 and 33, which, 

 though white, had eyes of normal size; there is good, reason to suppose that these specimens 

 were hatched in epigean waters, and that being carried into Lost River when young, the pigment 

 in its skin owing to absence of light had failed to assume its normal dark color. 



A parallel case is that mentioned by R. Schneider.* 



The author gives an account of the subterraneau variety of Gammarus pulex which is found at Clausthal. The 

 first point of interest is its pale color, pigment being so completely absent from its body that it is milk-white and 

 transparent ; even the fat-cells, which are intensely red or orange-yellow in the ordinary G. pulex, are quite white. 

 In the second place the eye is not normally developed, but is in the first stage of reduction ; the crystalline cones 

 show signs of degeneration, and the whole eye exhibits that "megalophthalmy" or proportionately greater size 

 which is often the first indication of loss. The pigment has also begun to be reduced, and is of a dirty black, instead of 

 a brownish color. The anterior pair of antenna) exhibit elongation, owing to the increase in the number of the joints. 



There is, as compared with the ordinary forms, a considerable increase in the amount of calcareous deposits; and 

 there is always a considerable amount of iron oxide in the contents of the intestine whence the iron makes its way to 

 various parts of the body. 



Friest suggests that experiments should be made on the effects of rearing normal, eyed Garn- 

 mari in darkness, and refers to Humbert's statement, that in the greater depths of Lake Baikal 

 with an increase in depth of their habitat there is an increasing lack of development of the eyes in 

 some Gammaridaj. Fries also states that he himself had previously observed a decrease in the 

 pigment of the eyes in young examples of Gammarus pulex living in darkness. 



Here should be cited the observations of Anton Stecker, who states that — 



Chernes, usually said to be eyeless, has rudimentary eyes, represented by clear, somewhat transparent spots, the 

 chitine forming them being devoid of the granulations covering the rest of the shield. Each cornea is supplied by a 

 large and well developed optic nerve, proceeding from an optic ganglion in connection with the brain. But the 

 layer of crystalline rods was wholly absent. About 30 to 35 per cent, of the specimens of Chernes cimicoides examined 



* Unterirdischo Gammarus von Clausthal. P. B. Ak. Berlin, 1885, p. 1087. Also: Abh. z. Programm k. Eeal- 

 Gymnaelmns Berlin, Ostern. Abstr. in: Journal Roy. Micr. Soc. (2), vi, p. 243. 

 t Zool. Anzeiger, Ang., 1879, pp. 36, 37. 



