Cordilleras and Archiguiana was reduced, these mountain streams, especially 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, Chœtostomus, 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 valleys, this system, particularly the Amazon, became progressively colonized from the older land areas and became the center of unparalleled adaptive radiation and a new center for distribution which it has remained to the present time. The comparatively few types inhabiting the old eastern land masses found themselves in possession of a continent 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 characins[1] of America.
From the Amazon species moved in all directions till they met barriers 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.[2] Others possibly crossed over the
- ↑ 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 dangerous 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 freshwater 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 Hydrocyninæ 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 remarkable family, Characidium fasciatum.
- ↑ See Science, N. S., XXII., pp. 18-20.
