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



between those of eastern and western Panama is in itself evidence of the 

 absence of subtrftpical species in this area. 



If the facts then are as stated, it remains for us to explain what we 

 may term this Panama 'fault' in the Colombian-Costa Rica subtropical 

 stratum of life. To one familiar with the influences governing the distribu- 

 tion of birds, the occurrence of so large a number of species, including many 



Fig. 18. Distribution of Atlapeles gutlwalis. A common species of the Subtropical Zone in 

 Colombia (A. g. guiiwalis) and from western Panama to Guatemala (A. g. hrunnescens) but which is 

 not known in the intervening area. 



of sedentary habit, at two such widely separated localities, is conclusive 

 proof that the localities themselves were connected. The fact that they 

 are joined by the comparatively low land between them has no bearing on 

 the case. The barriers which confine subtropical species to their zone are so 

 effective that these birds could no more cross the Tropical Zone separating 

 the areas they inhabit, than they could an intervening sea. Some of these 



