Page:Popular Science Monthly Volume 68.djvu/533

Rh 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 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. Others possibly crossed over the