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



form of each range is alike, all consequently differing from the Ecuadorian 

 form in the same manner; indicating, therefore, that their characters have 

 arisen independently and hence by parallelism. Examples are Ocreatus 

 underwoodi underwoodi and Boissoneaua flavescens flavescens. 



Usually, however, specific cases of variation in altitudinal distribution 

 show a tropical species ascending to the subtropics rather than the reverse. 

 Thus, Leptotila verreauxi mrreauxi is a form of the Tropical Zone in the 

 Magdalena Valley and eastward, but L. v. occidentalis in the Cauca region 

 is found in the Subtropical Zone. Columba svhvinacea berlepschi of the 

 tropics also grades into C. s. hogotensis of the subtropics. Lophotriccus 

 sqilamcBcrista squamcecrista inhabits the Subtropical Zone in all three ranges 

 of the Colombian Andes, but in southwestern Colombia and western Ecua- 

 dor i. s. minor is found at sea-level. Thryophilus nigricapillus connectens 

 is a subtropical form of the tropical T. n. schotti; Cacicus uropygialis uro- 

 pygialis is a subtropical form of C. n. pacificus. These are all intergrading 

 forms and as such evidently illustrate how subtropical species are derived 

 from tropical species. The inosculation of the upper limits of the Tropical 

 Zone with the lower limits of the Subtropical Zone make it difficult to under- 

 stand how, under existing conditions, these intergrading, representative 

 forms could become specifically separated. When, however, we attempt 

 to explain the origin of the numerous wholly distinct species and genera 

 now restricted to the Subtropical Zone we must take into consideration 

 the profound climatic changes caused by elevation of the Andes, and by 

 subsequent periods of glaciation which have produced wide fluctuations 

 in zonal levels. We must also consider points of origin and subsequent 

 zonal dispersion followed by complete geographic segregation from the 

 parent form. 



For example, the basal, or tropical ancestor of Rupicola peruviana is 

 possibly Rupicola rupicola; but the former has extended its range through 

 the Subtropical Zone to Bolivia, while the latter is restricted to the Guianan 

 region. Again, Pyroderus scutatv^ granadensis is now found in the Tropical 

 Zone of the Eastern Andes of Colombia, but P. s. scutatus, the probable 

 parent form, is found only in southeastern Brazil and eastern Paraguay. 



But until we have a far more detailed knowledge of the geological 

 history of the Andes and especially of the extent to which these mountains 

 have been glaciated, we shall not be in a position to discuss satisfactorily the 

 origin of its Subtropical Zone life. Meanwhile, as an ornithologist, I present 

 further data in regard to its Colombian elements. 



The sharply defined topography of the Andean system in Colombia 

 gives an equally clear definition to the zones of subtropical life which lie 

 on its slopes. In all three Andean ranges, north of Popayan, they are 

 widely separated below by Tropical Zone slopes and valleys, except at the 



