Todd-Carriker: Birds of Santa Marta Region, Colombia. 105 



and little circumstantial evidence to favor the view that the Subtropical 

 forms of the Sierra Nevada, as we know them today, originated where 

 they are now found from Tropical Zone stock, although there are a 

 few existing forms, the immediate relationships of which are not at 

 once evident, which may have originated thus. The fact that the 

 old coast range was in existence long before the rise of the Andes 

 seems not to have induced the development of a peculiar Subtropical 

 Zone fauna thereon, or if such an autochthonous fauna was developed 

 under these circumstances, no indubitable traces of it remain. In 

 brief, all the evidence points to the derivation of the fauna of the 

 Subtropical Zone of the mountain ranges of northern South America 

 from a more southern center of radiation by latitudinal extension. 

 It is easy to understand how this might have been if we assume (what 

 was certainly the case) that the Andean system was not uplifted simul- 

 taneously or equably. The first suitable areas raised to the necessary 

 height would have become the theaters of development for the new 

 and modified forms; as the elevation of adjacent areas continued and 

 new territory kept opening up, it would naturally be populated promptly 

 by extension from the original centers, which would of course be a 

 much more rapid process than the modification of the Tropical Zone 

 life of the region undergoing elevation to fit the new conditions. 

 Thus the newly rising areas would tend to become occupied by an 

 appropriate fauna as fast as they were raised. In this way we believe 

 that the forms of the Subtropical Zone have largely been evolved and 

 dispersed from a more limited center or centers of radiation. They 

 do not invade the Temperate Zone on the north or south for the 

 simple reason that the ground there is already occupied, and because 

 the conditions of humidity are not to their liking. 



The two upper life-zones, the Temperate and the Paramo, have 

 much in common, and may conveniently be considered together. The 

 Temperate Zone occupies the area between the levels of approximately 

 9,000 and 11,000 feet respectively, but some species obviously be- 

 longing to this zone are found as low down as 5,000 feet on the north 

 slope of the Sierra Nevada. The Paramo Zone similarly begins at 

 the upper level of the Temperate, extending thence to snow-line, but 

 there is a considerable overlapping in range between the species of 

 the two zones, due in part at least to the absence of true Temperate 

 Zone forest in this region. Both zones are greatlv restricted here, not 



