1917.] Chapman, Distrib,Mion of Bird-life in Colombia. 95 



map will at once show how much more of the earth's surface lies below 

 than above this level. Within its latitudinal limits the Tropical Zone may 

 therefore be spoken of as a sea of life in which the upper zones are mere 

 islands. 



The comparison fails, however, when one examines the conditions under 

 which life exists in the Tropical Zone, for instead of finding that uniformity 

 of aspect which characterizes the sea, we find a diversity of environment 

 far beyond that shown by any of the upper life zones. Shore-line, marsh, 

 savanna, llano, plain, and forest afford homes for a correspondingly wide 

 variety of forms, and, in connection with the extent of the area, go far to 

 account for the richness of its life. 



Returning again to our simile of the sea, when as in Colombia, the upper 

 zone islands assume the rank of peninsulas or are numerous enough to be 

 likened to archipelagos, more or less land-locked bays are formed which, 

 chiefly through their isolation, become centers for the development of new 

 types. 



All these characteristics of the Tropical Zone, as compared with those of 

 the zones above, are present in Colombia, and an attempt to define its 

 f aunal areas results in the recognition of no less than five more or less clearly 

 defined Faunas, as follows: 



1. The Colombian-Pacific. 



2. The Cauca-Magdalena, including both humid and arid sections. 



3. The Caribbean. 



4. The Orinocan. 



5. The Amazonian. 



Of these the first, or Colombian-Pacific, is the most important since in 

 connection with a high degree of humidity, not equalled elsewhere on the 

 tropical Pacific coast, or indeed in the western hemisphere, it combines an 

 isolation which has made it the principal local area of adaptive radiation 

 in Colombia.^ 



To a limited extent the Cauca-Magdalena region, both in its humid 

 lower Cauca and upper Cauca portions, has produced some new forms, and 

 the Caribbean Fauna, with its extension on the Venezuela coast, appears to 

 have been the birth-place of some of the species which are confined to it. 



'Orinocan' and 'Amazonian' are terms provisionally applied to the 

 respectively semi-arid and intensely humid portions of the Orinocan and 

 Amazonian basins, which find their western borders at the base of. the East- 

 em Andes. They are merely small parts of much larger faunas and possess 

 no distinctive features of their own. With these general remarks on the 



1 Cf. Osborn, The Age of Mammals, 1910, p. 22. 



