92 Bulletin American Museum of Natural History. [Vol. XXXVI, 



The whole question of the origin of Andean life-zones is, from the orni- 

 thological point of view at least, a new one, and I cannot at this stage of 

 our knowledge hope to do much more than open it for discussion. 



The fundamental factors in zone formation, and the conclusions reached 

 from our studies in Colombia, may be stated as follows : 



Factors. 



Past 



Rise of the Andes 

 Glaciation 

 Erosion 

 Subsidence 

 [ Temperature 

 Present i Humidity 



[Soil 



Environmental factors. 



Corwlusions. 



1. Existing faunal conditions in the Tropical Zone are, in part pre- 

 Andean, in part post-Andean. The humid Pacific coast, for example, 

 contains many species which appear to have occupied this region prior to 

 the Andean uplift. The life of the Cauca Valley, on the other hand, seems 

 to be of post-Andean origin, its analysis supporting the geological evidence 

 that this valley was occupied by a lake until post-tertiary time. 



2. The Subtropical Zone nowhere reaches sea-level. Its life has been 

 derived from the Tropical Zone. Because of its greater age and because 

 altitudinal extension of range may imply greater environmental change 

 than occurs in latitudinal extension, its life varies more widely from that 

 of the ancestral area than does the life of the remaining two zones. 



3. When a Colombian Subtropical Zone form differs from its Ecuadorian 

 representative, there is usually one race in the East Andean Faima and 

 another in the West Andean Pauna. In some instances, however, the 

 Colombian form is alike in all three branches of the Andes, though these 

 three arms of its range are separated by the intervening tropical areas, 

 indicating that the same characters have been developed in the individuals 

 of each mountain chain by parallelism. 



4. Receding glaciers, erosion, and subsidence have produced zonal 

 islands and zonal 'faults.' The distributional evidence on which, for 

 example, the Panama 'fault' is shown to have occurred indicates that the 

 Andes of Colombia and the higher mountains of western Panama and Costa 



