ADAPTATIONS 399 



which closely resemble Amblyopsis, and they are separated 

 from the Cyprinodontidas as a distinct family, Amblyopsidae, 

 differing from the former chiefly in two characters, namely 

 that the mouth is scarcely at all protractile and that the anus is 

 placed far forward close to the gills. The pelvic fins are rudi- 

 mentary or absent, but this is also true of some Cyprinodonts. 

 The family consists of three genera, Chologaster, Typhlichthys 

 and Amblyopsis. In the first of these both eyes and pigment 

 are well developed. Chologaster cornutus is abundant in the 

 Dismal Swamp of Virginia and other swamps and rice-fields 

 southward to northern Florida. It is less than two inches long, 

 is destitute of pelvic fins, and is striped with longitudinal bands 

 of black colour. Chologaster papilliferus is found under stones 

 in the rivulets of Illinois, and Chologaster agassizii occurs in the 

 underground streams of Kentucky and Tennessee. Typhlich- 

 thys is closely allied to Chologaster but is blind and colourless ; 

 it lives entirely underground in the limestone caves of southern 

 Indiana, and thence southward to the north of Alabama. The 

 fact that Chologaster agassizii is coloured and has well-developed 

 eyes, although it lives in the dark, doubtless is sufficient to con- 

 vince some biologists that the loss of eyes and colour in Typh- 

 lichthys is not due to the absence of light ; but the fact is as 

 puzzling on the selection theory as on the other. It is difficult 

 to believe that constant residence in the dark does not at least 

 reduce the pigmentation of Chologaster agassizii in the indiv- 

 idual without any reference to heredity, but in any case it is 

 obvious that Typhlichthys has become blind and colourless in 

 consequence of living in the dark. It is probable that Typhlich- 

 thys has been subjected to the conditions of subterranean life 

 for a much longer period than Chologaster agassizii, and that the 

 latter will lose eyes and colour in course of time if confined 

 generation after generation to a subterranean habitat. 



Amblyopsis spelcea differs from Typhlichthys in its greater 

 size, reaching a length of 5 in., and in the possession of small 

 pelvic fins ; thus although there can be no doubt that Typhlich- 

 thys is a modified form of Chologaster, which has lost eyes 

 and colour in consequence of its subterranean life, the normal 

 or ancestral form of Amblyopsis is unknown. It is confined to 

 the east side of the Mississippi ; in caves to the south of the 

 Ohio both Typhylichthys and Amblyopsis occur together, to 



