210 BLIND VERTEBRATES AND TfiEIR EYES. 



Still more striking is the variability in the size of opposite eyes in the same 

 individual whatever its length. There are minute differences in the size of the eyes 

 of the two sides at all times, the individual with two eyes exactly alike is probably 

 not to be found, but the differences in mind are of a much larger order. For in- 

 stance, in the mother of the 4 young mentioned above, the left eye had a longitudinal 

 diameter of 170, the right eye 225 ; that is, the right eye was a third longer than the 

 left. Instances of this sort are by no means rare, there being a marked difference in 

 a number of the individuals secured. In one of the oldest secured, the eye of one 

 side is all but gone, that of the other still well defined (plate 23 and plate 24, d). 

 In a much younger one, 43 mm. long, I have found no eye in one side. In another 

 the left eye bears the ratio of 1 to 3 to the right eye, which is therefore almost 

 nine times as large as the left (plate 24, figs. A and b). 



Such big differences between the eyes of the two sides, fluctuating in amount 

 in different individuals, but readily seen in living specimens, are found in about 10 

 per cent of individuals. Sections usually showed that such differences whenever 

 they existed were largely to be found in the pigment layer which in the large eyes 

 was vesicular and the retina shriveled and retracted to the pupil, leaving a large 

 space between the pigment epithelium and the rest of the retina (plate 21, fig. b; 

 plate 22, fig. a; plate 24, figs. A and c). 



Note. — One element of error is present in the exposition of the eye of Lucifuga. Lucifuga and 

 Stygicola live together in the same caves. There is no difficulty in distinguishing these after they 

 reach a certain size. What that size is I can not say, but at 60 mm. they are conspicuously different. 

 The smallest specimen of Stygicola unquestionably determined is 60 mm. in length. Possibly 

 the two species are superficially indistinguishable when young, and some of the young specimens 

 mentioned below 60 mm. and used in preparing the following account may in reality be stygicolas. 

 All specimens below 60 mm. secured had the characters of Lucifuga. The probability of this pos- 

 sible error is not as great as it may appear at first sight, as an analysis of the origin of the specimens 

 less than 60 mm. will show. Seven of the specimens less than 60 mm. sectioned are from the cave 

 of Jaiguan. From this cave 23 fishes were taken, 5 of which were stygicolas. The smallest of the 

 stygicolas was 81 mm. and considerably larger than the smallest undoubtedly distinguishable spec- 

 imens. If there were no specimens of Stygicola between 60 mm. and 81 mm. long when they 

 could have been readily distinguished, it is probable that there were none smaller. The 3 smallest 

 specimens of Stygicola measured 81, 90, and 97 mm. respectively. From Hawey I secured only 

 Lucifuga, at least 3 of them being larger than the smallest specimens, permitting an unquestioned 

 determination. From La Fria the only 2 over 60 mm. long were Stygicola, while those below 57 mm. 

 were apparently all lucifugas. Two of those sectioned, 54 and 57 mm. long, may be considered 

 lucifugas without a doubt. This leaves one 27 mm. and one 28 mm. in doubt. In Los Bafios we 

 secured no large specimens ; all the small ones were referred to Lucifuga. In Ashton large and small 

 were all referred to Lucifuga, the smallest one sectioned from this place being 53 mm. ; it is undoubt- 

 edly a Lucifuga. 



The proportion of stygicolas to lucifugas among individuals over 60 mm. is: stygicolas, 43; 

 lucifugas, 36. Lucifuga does not reach a size over 104 mm., and comparing the ratios of lucifugas to 

 stygicolas, between the smallest determined Stygicola 60 mm. and the largest Lucifuga 104 mm., we 

 get Stygicola 32 mm., Lucifuga 36 mm., or a ratio of 1 to 1.25. But of the 32 stygicolas between 60 

 mm. and 94 mm., 10 came from the "M " Cave which is remote from the region where lucifugas were 

 found. Eliminating these, we would get a ratio of 22 to 36, or 1 to 1 \, for the region where both 

 a e found. This, other things being equal, would give us the probability that any of the 

 younger specimens found in the region where both species were found was a Stygicola or a Luci- 

 fuga. More than this, in the "M" Cave, about 60 miles removed from any cave in which Luci- 

 fuga was found, Stygicola is very abundant, but we secured no specimens less than 60 mm. long in 

 five trips, nor were any small ones found in the Donkey and Carboneria, where only Stygicola 



