1917.] Chapman, Distribution of Bird-life in Colombia. 93 



Rica were, until comparatively recent geologic times, connected by a range 

 having an altitude of not less than five thousand feet. 



5. The Temperate Zone reaches sea-level in the South Temperate Zone. 

 Its life is derived in part by zonal, in part by latitudinal extension and is 

 more recent than that of the Subtropical Zone. 



6. The Paramo Zone reaches sea-level in southern South America. 

 Its life is derived by altitudinal extension and is more recent than that of 

 the Temperate Zone. 



7. The present trend of the distribution of life is northward. Few 

 boreal species have entered Colombia in recent geologic times. 



8. With rare exceptions (e. g., Brachysjdza capensis peruviana) no 

 species extends its range from an upper to a lower zone. 



9. Wide latitudinal range usually implies wide altitudinal range. 

 10. Uniformity of life increases with altitude. 



The Teopical Zone and its Faunas. 



The Tropical Zone in Colombia occupies all that part of the country 

 lying approximately below an altitude of 5000 feet. In some few places 

 it does not extend much above 4500 feet; in others it reaches to about 

 6000 feet. Its limits are determined primarily by temperature, but they 

 are further dependent upon humidity, as humidity itself affects temperature 

 through radiation. 



On the heavily forested Pacific slope of the Western Andes the lack of 

 favorable radiating surfaces is conducive to a lower temperature than is 

 found at a corresponding level on the barren eastern slopes of the same range. 

 In consequence, the upper margin of the Tropical Zone is at least 1000 feet 

 higher on the eastern than on the western side of these mountains. 



Unfortunately no exact data on temperature are available in this con- 

 nection, but the influence of radiation was observed in a marked manner 

 in the succeeding or Subtropical Zone at the San Antonio Pass (alt. 6800ft.) 

 where the road from Buenaventura to Cali crosses the Western Andes;- 



With a regularity which has given it the name of the "San Antonio 

 Wind," at two o'clock each day a strong, cold, westerly wind, usually with 

 a driving mist, sweeps over the crest of the range. 



This phenomenon is apparently attributable to decreasing barometric 

 pressure following radiation from the comparatively open floor of the Cauca 

 Valley, when, already urged by the prevailing westerly wind, air from the 

 coast rushes into the area of lower pressure and is condensed as it reaches 

 the higher parts of the range. 



