156 Bulletin American Museum of Natural Hsitory. [Vol. XXXVI, 



at which time this zone stretched more or less continuously from north- 

 western Colombia through Panama to Costa Rica. 



If this be true, it follows that the mountain system of Panama must 

 have had an elevation of not less than five thousand feet. The ornithologi- 

 cal evidence also indicates that this range could not have had a greater 

 elevation than nine thousand feet. This statement anticipates a treatment 

 of the Temperate Zone, which, as will be shown, is present at the northern 

 end of the Western Andes, above an altitude of 9000 feet. The higher 

 mountains of western Panama and Costa Rica also exceed this altitude but 

 there is no such close relation between the bird-life of the Colombian and 

 Costa Rican Temperate Zones as exists between that of their Subtropical 

 Zones. 



The Temperate Zone avifauna of Costa Rica, so far as one can trace its 

 origin, appears to have been derived largely from the north, while that of 

 Colombia has been derived chiefly from the south. 



We apparently are therefore justified in assuming that there has been no 

 actual connection between the Temperate Zones of these two regions and 

 this, in turn, implies that the intervening mountains have not exceeded an 

 elevation of 9000 feet. 



Subsidence, of which there is abundant geological evidence, and erosion 

 are the factors which have doubtless brought the mountain system of 

 Panama below the subtropical level, except at its eastern and western 

 extremities. In the effort to picture to ourselves this region as it existed 

 when, as we believe, the Subtropical Zone extended from Colombia to Costa 

 Rica, we may try also to imagine the fate of those individuals of representa- 

 tive subtropical species which inhabited that part of the zone which sub- 

 sequently disappeared. As their home gradually contracted in extent, only 

 two lines of retreat were open to them. They could go downward into the 

 Tropical Zone, or they could go horizontally toward that part of their zone 

 which had not sunk into the zone below. 



Species of the Tropical Zone extend their range into the Subtropical 

 Zone, but I recall only one instance of the reverse occurring. Not only are 

 upper zone forms held by those environmental bonds which determine 

 their zone, but the causes which prevent an island form from taking root 

 on the mainland doubtless also prohibit a bird from extending its range to a 

 zone below the one it occupies. In both instances the ground is occupied. 

 So we find none of the species which we believe to have occupied the former 

 Panama Subtropical Zone in the Tropical Zone of that country. 



Successful retreat toward either end of the disappearing zone would be 

 even more diflficult than to the zone below, for here the resulting over- 

 crowding would bring them into competition with species of similar habits. 



