FISHES OF SOUTH AND MIDDLE AMERICA 529 



Cordilleras and Archiguiana was reduced, these mountain streams, espe- 

 cially those of Ecuador and Colombia, became populated by stragglers 

 or accidental visitors from the land areas to the east. These in their 

 turn, with the elevation of the Andes, became modified and gave rise to 

 the genera now peculiar to both slopes of the high Andes, Pygidium, 

 Eremophilus, Chcetostomus, Arges, Cyclopium, Astroblepus, etc. 



With the further elevation of the Cordilleras into a continuous 

 barrier and the formation of the Orinoco, Amazon and La Plata valleys 

 through elevation and the debris brought from the land masses and the 

 development of the enormous fresh-water system occupying these val- 

 leys, this system, particularly the Amazon, became progressively colo- 

 nized from the older land areas and became the center of unparalleled 

 adaptive radiation and a new center for distribution which it has re- 

 mained to the present time. The comparatively few types inhabiting 

 the old eastern land masses found themselves in possession of a con- 

 tinent and diverged in every conceivable direction. I have hinted at 

 this divergence in a recent article (Biol. Bull., VIII., pp. 59-66). It 

 will be considered in detail in a forthcoming monograph of the char- 

 acins 3 of America. 



From the Amazon species moved in all directions till they met bar- 

 riers of one sort or another. The Pacific slope fauna is derived to a 

 very large extent from this later divergent migration over the Isthmus 

 of Panama and through the valley of the Atrato, between the western 

 and coast Cordilleras of Colombia. 4 Others possibly crossed over the 



3 The characins are a family of fresh-water fishes that, in America, range 

 from the border of the United States to some distance south of Buenos Aires. 

 They form about one third of the entire South American fresh-water fauna, 

 and have diverged in adaptation to diverse food, diverse habitat and diverse 

 enemies to fill nearly every niche open to fishes. The ends of the three lines 

 of adaptation to different food give us mud-eating forms, with long intestinal 

 tract and no teeth; flesh-eaters with shear-like teeth, that make bathing dan- 

 gerous to life and that cut their way out of nets; and conical-toothed forms, 

 with sharp, needle-like teeth and comparatively huge fangs. Greater diversity 

 could scarcely be imagined, and one is lead to suspect that some of the forms 

 are over-adapted. In their divergence in form they have reached almost every 

 conceivable shape. . . . 



Diverging among themselves as has been noted above, they have approached, 

 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 paralleled by 

 Salminis and Catabasis; our minnows 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 Hydrocynina to our gar pikes; our mullets 

 are duplicated by Prochilodus : our top-minnows are mimicked by Nannostomus, 

 and even our festive darters are duplicated by a member of this most remark- 

 able family, C'haracidium fasciatum. 



4 See Science, N. S., XXII., pp. 18-20. 



vol. lxvui. — 34. 



