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



Where such an upHft created a mountain system as continuous as the 

 Andes now are, these new areas were doubtless populated by latitudinal 

 extension of range from regions having similar climates, and by altitudinal 

 extension as the pressure of life from immediately contiguous regions below 

 forced species upward, the more adaptable of which survived. 



Although having the shorter journey, the change of environment would 

 be greater for those species coming from another zone in the same latitude 

 than for those coming from the same zone in a perhaps distant latitude. 

 We should, therefore, expect to find greater variation in what may be called 

 zonal representatives than in altitudinal representatives. 



A study of the existing fauna supports these theories of the origin of 

 zonal life and the degree of variation it presents. Thus the birds of the 

 Subtropical Zone have been almost wholly derived from the zone below; 

 those of the Temperate Zone came in part from the Subtropical, in part from 

 the same zone at sea-level, while nearly all those of the Paramo Zone have 

 come from the sea-level equivalent of this zone in southern South America. 



It follows, then, that the birds of every zone above the tropics have been 

 derived from a lower level. There are some exceptions to this rule but they 

 do not affect the general truth of the statement. In comparative varia- 

 bility the fauna of the Subtropics differs more from the ancestral stock in 

 the tropics than do the altitudinal forms of the Temperate and Paramo 

 Zones from their distant sea-level derivatives of the South Temperate Zone, 

 with which indeed they are often specifically identical. Hence it follows 

 that uniformity of life increases with altitude, while as a corollary, the 

 number of species decreases; uniformity of environment being apparently 

 the underlying cause. 



The sometimes marked difference in the character of alluvial bottom- 

 lands and slopes arising from them, even when both are wooded, exerts a 

 strong influence on the range of some species of the Tropical Zone. Certain 

 terrestrial birds, like Pittasoma, for example, are confiined to the muddy 

 shores of slow-flowing streams. Others, like Opisthocomus, do not leave 

 the growth along the borders of such streams. Still others frequent the 

 floor of the lowland forest. 



Such restrictions of range, however, appear to me to be of habitat rather 

 than of zone, and do not, in my opinion, requirca subdivision of the Tropical 

 Zone. 



We obtained no evidence of altitudinal migration among Colombian 

 birds, though it is probable that Hummingbirds range up and down moun- 

 tain sides in search of certain flowers. 



We cannot of course expect to find conclusive evidence of the geographic 

 origin of all the species of, a given zone. Possibly the ancestral forms and 



