DIVERGENCE AND CONVERGENCE IN FISHES. 6 1 



The characins again furnish striking illustrations. Diverging 

 among themselves, as has been noted above, they have ap- 

 proached, or paralleled many members 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 paralled by Salininus and Cata- 

 basis ; our minnows are paralleled by Tetragonopterus and its re- 

 latives. It will take but a slight flight of the imagination to 

 detect the striking similarity of some of the Hydrocyninae to our 

 gar pikes ; our mullets are duplicated by Procliilodits ; our top- 

 minnows are mimicked by Nannostomus, and even our festive 

 darters are duplicated by a member of this most remarkable 

 family, Characidium fasciatinn. 



I have elsewhere given an example of convergence under the 

 similar conditions presented by different caves. TroglicJitliys 

 rosce, a blind fish of southwestern Missouri, resembles so closely 

 Typhlichthys subterraneus of Kentucky that at least two natural- 

 ists, expert in detecting specific differences in fishes, had failed to 

 distinguish them, although below the adaptive resemblances of 

 the surface there was distinct evidence that the epigean ancestors 

 of the two species were generically distinct. 



A cave presents a " unit of environment," a little world, a mi- 

 crocosm, with both positive and negative characters. 



1. This unit is well circumscribed and connected with the rest 

 of the universe frequently by but a single narrow vestibule. In 

 this vestibule we have a rapid graduation from external to inter- 

 nal cave conditions - - a region which acts as a purgatory for all 

 forms that would enter the elysian fields within. The majority 

 of epigean forms never get beyond the twilight of this region. 



2. The feature that distinguishes the interior of the cave is the 

 constant absence of sunlight and all which that implies. 



(a\ The absence of all green plants and consequently the ab- 

 sence of all direct food-producing activity, (b] As a result of (a] 

 all food must ultimately be imported. (V) As a result of (b) the 

 abundance of the fauna in different caves is frequently inversely 

 proportionate to the size of the cave, or directly to the ability of 

 the openings to admit food to the various recesses of the different 

 caves. 



