1917.] Chapman, Distribution of Bird-life in Colombia. 127 



largely into the Cauca Valley, a comparatively arid district; but the tropical 

 life of the valley, on the other hand, has crossed the Subtropical Zone of 

 the Western Andes and entered a favorable environment on its western 

 slope. 



It should be noted, however, that on the treeless lower slopes of the 

 eastern side of the Western Andes, the Tropical Zone extends to a greater 

 altitude than it does on the humid western slopes. At San Antonio and 

 Cresta de Gallo, for example, it practically reaches the divide from which 

 one can almost see the Dagua basin, the upper margin of which lies not more 

 than 1200 feet below. Only a small barrier, therefore, prevents the passage 

 of species from the Cauca Valley to the upper Dagua Valley. 



On the western slopes of the Western Andes, the humid Tropical Zone 

 does not attain so great an altitude as does the arid Tropical Zone on its 

 eastern side, and the Subtropical Zone to be crossed is correspondingly 

 wider. 



In spite of its isolation from other regions possessing similar character- 

 istics, the Cauca Valley has given rise to but few geographical forms, and 

 this fact in connection with its apparently limited life suggests that the 

 existing fauna has been acquired at a comparatively recent date. 



It had occurred to me that possibly the floor of the Cauca Valley is an 

 ancient lake-bed but with no geological evidence to support this theory, I 

 had hesitated to advance it, but on re-reading Robert Blake White's ' Notes 

 on the Central Provinces of Colombia' (Proc. R. G. S., V, 1883, p. 250) after 

 the preceding observations had been written, I find this exceedingly inter- 

 esting statement: "Directly to the eastward of this group [Supia and 

 Tado Moros] of igneous rocks lies the great volcanic centre of Herveo, 

 Tolima and Santa Isabel, and there can be no doubt that the valley of the 

 upper Cauca was for some time in the post-tertiary period converted into a 

 lake, owing to the upheaval of the flanks of the volcanoes mentioned. 

 However, their action also produced a fracture parallel to the opposing 

 western cordillera, and the waters of the Cauca at last worked their way 

 northwards and now run through one of the grandest ravines imaginable." 



Here then, we have an apparently satisfactory explanation of the charac- 

 ter of the Cauca Valley fauna, which appears indeed to be of post-Andean 

 origin. 



List of Species and Svhspecies which Characterize the Humid Cauca-Magdalena Fauna. 



Crax alberti Capito hypoleucus 



Amazona salvini Ramphastos oitreolffimus 



Pyrilia pyrilia Brachygalba satooni 



Momotus subrufescens subrufescens Campephilus malherbi 



" reconditus Thamnophilus nigriceps 



