DIVERGENCE AND CONVERGENCE. 19 



gobies and blennies that occupy a similar position along the marine shores has 

 repeatedly been noticed. 



In the tropics live many burrowing lizards and snakes. Rhineura, one of the 

 lizards, lives and acts like an earthworm, and so like an earthworm has it become 

 that only a close inspection reveals its true nature. Even the chickens following 

 the plows in Florida and Cuba are said to be taken in by the similarity of some 

 of the burrowing lizards to earthworms. 



The Characins again furnish striking illustrations. Diverging among them- 

 selves, as has been noted above, they have approached, or paralleled, many mem- 

 bers of the diverse families of North American fresh-water fishes. Our shads and 

 fresh-water herrings have their counterparts in Elopomorphus, Potamorhina, and 

 Psectrogaster ; our salmon are paralleled by Salminus and Catabasis ; our min- 

 nows are paralleled by Tetragonopterus and its relatives. It will take but a slight 

 flight of the imagination to detect the striking similarity of some of the Hydro- 

 cyninae to our garpikes; our mullets are duplicated by Prochilodns ; our top- 

 minnows are mimicked by Nannostomus ; and even our festive darters are dupli- 

 cated by the species of Characidium, members of this most remarkable family. 



In a dark cave, all those differences between related species which would strike 

 the eye, such as protective coloration, recognition marks, decorations of any sort, 

 etc., are absent, and related species tend to look alike. It was not until after 

 a detailed examination of many specimens that I could invariably distinguish 

 Luclfitga and Stygicola, the Cuban blind fishes, from each other. 



On the surface the specimens of Troglichthys rosce very closely resemble Typh- 

 lichthys subtcrranms from Mammoth Cave, differing slightly in the proportion and 

 in the pectoral and caudal fins. These fins are longer in rosa. It is, however, 

 quite evident from a study of their eyes that we have to deal here with a case of 

 convergence of two very distinct forms. They have converged because of the 

 similarity of their environment and especially owing to the absence of those ele- 

 ments in their environment that lead to external protective adaptations. It would 

 be difficult to distinguish specimens of similar size of Amblyopsis from either 

 subterrancus or rosce, were it not that it possesses ventrals. 



The eye of T. subterraneus is surrounded by a very thin layer of tissue repre- 

 senting the sclera and choroid. The two layers are not separable. In this re- 

 spect it approaches the condition in the epigean, eyed member of the family, Cholo- 

 gaster. For other reasons, that need not be given here, it is quite certain that 

 Typhlichthys is the descendant of a Chologaster. The intensity of coloration and 

 the structure of the eye are the chief points of difference. The eye of rosce is but 

 about one-third the diameter of that of subterrancus, measuring 0.06 mm. or there- 

 about. It is the most degenerate, as distinguished from undeveloped, vertebrate 

 eye. The point of importance in the present instance is the presence of com- 

 paratively enormous scleral cartilages. 1 These have not degenerated in propor- 

 tion to the degeneration of the eye and in some cases are several times as long as 

 the eye, projecting far beyond it, or are puckered to make their disproportionate 

 size fit the vanishing eye. This species is unquestionably descended from a species 

 with well-developed scleral cartilages, for it is not conceivable that the sclera as 

 found in Chologaster could, by any freak or chance, give rise during degeneration 



1 Kohl mistook the nature of these structures, as he did of every other connected with these eyes, except the 

 lens and ganglionic cells. 



