EIGENMANN: the freshwater fishes of BRITISH GUIANA 107 



watwata of the lower courses. Moenkhausia browni replaces Moenkhausia grandi- 

 squamis, Creatochanes affinis replaces Creatochanes melanurus, Chalcinus rotundatus 

 replaces Chalcinus elongatus, Charax rupununi replaces Charax gibbosus, Pimelodella 

 cristata and P. megalops replace P. madurki. 



In a number of other places the replacement is not so conspicuous. In several 

 of the above mentioned cases the species overlap. In some of the cases there is 

 quite evident genetic connection between the two mutually excluding forms. In 

 other cases the genetic connection, while evident, is certain y remote, as in the 

 case of Crenuchus and Poecilocharax, for instance, and in still other cases the relation 

 may be apparent rather than real, i. e., one or both forms may have immigrated to 

 adapted localities, and not have become differentiated in their present habitat. 



In so far as two of the above pairs represent local adaptations, that is, in so 

 far as one or both members of the pair are autochthons, thus far do they give evi- 

 dence in support of Jordan's Law. Where, however, we find such cases in support 

 of the law, there are many others where the more closely related species occupy the 

 same territory and are contrary to it. We may cite Phenacogaster megalostictus and 

 microstictus; Chasmocranus longior and brevior; Doras hancocki and cataphrada; Curi- 

 matus microcephalus and spilurus; Characidium pellucidum, C. pteroides and C. cate- 

 natum; PcBcilobrycon trifasciatus, P. ery thrums and P. ocellatus. It may be said in 

 favor of Jordan's Law that in some of the cases in which we are certain that the 

 forms are immediately related {Crenicichla and /Equidens) its postulates are 

 undoubtedly sustained. 



Mutation. 



Two instances of apparent mutation may be mentioned. 



L Moenkhausia profunda differs from Fowlerina orbicularis apparently only 

 in the generic characters. I say apparently, for it is to be borne in mind that in 

 comparing "specimens" of fishes we are put on a level with the ornithologist who 

 might be compelled to compare plucked specimens of the Orchard and Baltimore 

 Orioles. It is doubtful whether he would draw correct conclusions as to their song 

 and nesting-habits from such data. Similarly Mcenkhausia profunda and Fowlerina 

 orbicularis may differ from each other much more than the characters of speci- 

 mens seem to indicate. Granting that the two species are as similar as they appear 

 to be it is quite probable that the former is a recent mutant of the latter. Fowlerina 

 is exceedingly abundant in the Essequibo. Moenkhausia profunda is known by but 

 two specimens from the northwestern coast. (Plate XLVI, figs. 1 and 2.) 



2. The second instance is complex and indicates simultaneous, orthogenic 

 mutation in a number of not closely related species of the same family. 



