1917.] Chapman, Distribution of Bird-life in Colombia. 87 



as more appropriate, is somewhat less extensive altitudinally than in Colom- 

 bia, and the fourth, or Paramo, is correspondingly larger. Local conditions, 

 including the much greater superficial area of the Paramo Zone in Ecuador, 

 and the isolation of the temperate interandine region from Subtropical 

 influences, are no doubt responsible for these differences. 



Wolf's zones, as well as the comparatively simple topography of the 

 Andean system in Ecuador, are shown in the accompanying diagram from 

 his standard work {I. c, p. 441). 



The extent, general, and ornithological characters of the zones herein 

 proposed are presented in detail beyond, but here I offer several general 

 considerations in regard to Andean zonal life as a whole. 



Any attempt to explain existing conditions must be preceded by an 

 effort to picture to ourselves the effect on the fauna of a tropical region of 

 the uplift in it of a mountain system to snow-line. If at some point in the 

 heart of the humid tropics, let us say upper Amazonia, progressive cooling 



Fig. 2. Ideal section through the Ecuadorian Andes to show zones of vegetation. 



2. Llanos. 3. Tropical and Subtropical Forests. 4. Interandean Region [ = Temperale Zone]. 



5. Andean Region [ = Paramo Zone]. 6. Perpetual Snow. (From Wolf, 1892.) 



should eventually produce a snow-covered area surrounded by successive, 

 concentric, climatic belts leading gradually to the surrounding tropics, we 

 should have no more striking climatic change than has been brought about 

 by the elevation of the Andes. 



Geologists, I believe, are agreed that this great mountain system is of 

 Tertiary origin, and that there have been pronounced uplifts as late as the 

 Pleistocene. Perhaps, therefore, we are warranted in assuming that the 

 range had not acquired sufficient elevation to become an effective barrier 

 to the distribution of tropical life prior to the latter half of the Tertiary. 



However this may be, one's imagination is stimulated by an attempt to 

 follow the course of events as a gradually increasing elevation, with its 

 subsequent changes, brought into existence new habitable areas of the earth's 

 surface with strikingly different climates from that of the base out of which 

 they had arisen. 



