•21G Mr. \y. F. tie Vismcs Kane on 



wliich tliese and similarly situated animals display lias been 

 tlie subject of much discussion among biologists, of whom 

 Packard and Semper may be mentioned, some holding that 

 the effect of disuse of the organ has brought about its parti d 

 or total degeneration, others that in localities where light 

 prevails the individuals with perfect vision would survive those 

 with defective sight, and that natural selection would secure 

 the maintenance of these valuable organs ; but that where 

 darkness makes the possession of sight in no way advan- 

 tageous to the possessor the eyed and eyeless forms would 

 breed together and have equal chances in the struggle of 

 life, with the result that every degree of atrophy of the parts 

 cf tiie eye might be expected to supervene till a totally blind 

 race resulted. 



Examjjles illustrating each stage of this degeneration have 

 been discovered in such situations. Leydig, writing of the 

 blind JSi/>hargiis subterraneus, mentions the fact that some 

 specimens have been observed with pigment-spots in the 

 region of the eye, and states that he has found that though 

 the optic ganglion is present the eye is wanting. Cliilton 

 remarks of the subterranean blind species of New Zealand 

 that in only one species has he discovered any external 

 traces of eyes, namely two or three small lenses without 

 pigment*, and he thinks it probable that in different species 

 there may be great differences in the amount of degeneracy 

 of the eyes, and perhaps also in individuals from different 

 localities. 



A lake form of the common Gammarus pulex at Wiiden- 

 schwyl was found by Prof. Asper to have both blind and 

 eyed terras at a depth of 40 metres. Similar facts have been 

 adduced by other authors in regard to lake varieties of the 

 Isopod Asellus &c. Packard, in his description of the cave- 

 fauna of North America, mentions that in the genus Gecidokta 

 one species is without even a vestige of an eye, though in 

 others there are imperfect traces. It is therefore interesting 

 that among about 130 specimens of iSiphargus Kochianus 

 from Lough Mask 1 found three which had well-developed 

 pigmentation, and a fourth which showed a cloudy shading 

 in the optic region. But, in the absence of opportunity to 

 have careful sections made by a competent zoologist, it is 

 impossible to say how far the structure of the eye may be 

 preserved in these specimens. The absence of colour is like- 

 wise another general characteristic of subterranean fauna. 



* " Subterranean Crustacea of New Zealand," C. Chilton, Trans. Linn. 

 Soc, 2nd ser. Zool. vol. vi. pt. 2, p. 220. 



