24 BuHetin American Museum of Natural History. [Vol. XXXVI, 



Andes is therefore surrounded by forest and the character and origin of its 

 fauna is hence of much interest. It apparently cannot be derived from the 

 humid, heavily wooded slopes above or below it, and the height of the 

 mountains to the east is presumably sufficient to separate it from the 

 faunally similar Cauca Valley. Nevertheless, its bird-life has evidently 

 been derived from that valley. When, however, one observes that owing 

 to the aridity of the eastern slope of the Western Andes the Tropical Zone 

 ascends nearly to the San Antonio pass, it is clear that the Tropical Zones 

 of the Caldas and Cauca Valleys are separated only by the narrow belt of 

 timber which crowns the San Antonio pass. Hence we have numerous 

 Cauca Valley species occurring at Caldas but apparently not elsewhere on 

 the Pacific slope in this section. 



Caldas to San Antonio. — At Caldas the trail leaves the banks of the 

 Dagua and winds gently up the slope toward the San Antonio pass. At an 

 altitude of 5700 feet we entered the clouds and, at the same time, the lower 

 border of the cloud forest which characterizes the Subtropical Zone. The 

 Caldas region now appeared as a treeless depression surrounded by forest- 

 crowned mountains. Everywhere the tree-line was as sharply defined as in 

 a fresh clearing." The cloud-line coincided with the tree-line. Cloudless 

 hilltops were bare of trees. 



The luxuriant forest of the Subtropical Zone continues to the summit 

 of the ridge and as far over it as the cloud's-cap itself. Normally, this is 

 not more than a few hundred feet, but when ravines or barrancas slope 

 down toward the Cauca Valley the water they carry leads the forest to a 

 much lower level than it reaches without the encouragement of such natural 

 irrigation. These wooded barrancas are separated by grass-grown ridges 

 of the treeless eastern slope of the Western Andes. These ridges carry a 

 limited number of species of the Tropical Zone upward almost to the San 

 Antonio pass, just as the forest's arms stretching down the barrancas carry 

 some Subtropical Zone species well below the upper limits of the Tropical 

 Zone. The result is an inosculation of faunas occasioned by causes which 

 are obvious enough when seen, but which the most accurately labeled 

 specimens would not reveal. 



The crest of the range is here so narrow that the descent into the Cauca 

 Valley begins almost where the ascent from the Pacific ends. One has to 

 go only a few hundred feet below the divide to pass from the forest into a 

 low, scrubby growth which quickly gives Way to the brown, treeless slopes 

 leading down to the Caiica Valley. 



Most of our collecting in this vicinity was done in the forests, but occa- 

 sionally work was done along its border and here certain tropical species 

 were secured, a fact which accounts for their being recorded from a locality 

 whiqh in reality is in the Subtropical Zone. 



