1917.] Chapman, Distribution of Bird-life in Colombia. 89 



point of origin of only those it has acquired most recently may be determina- 

 ble, while the derivation of the earlier arrivals may forever be unknown to us. 

 Consequently we have a large group of species whose history is lost to us 

 and which, therefore, we can only assume have entered their zone under the 

 influences which are still active, and the cumulative effects of which they 

 exhibit. 



Aside from creating areas where, under the influence of a new environ- 

 ment, evolution has evidently proceeded at a highly accelerated pace, the 

 topographic changes incident to the elevation of the Andes have profoundly 

 affected the distribution of life in the Tropical Zone. 



A comparison of the bird-life of the Pacific coast of Colombia and north- 

 ern Ecuador with that of the Tropical Zone at the eastern base of the Andes 

 in southeastern Colombia and eastern Ecuador, induces the belief that we 

 have here, in part, a pre-Andean fauna, the Pacific portion of which has been 

 cut off from that of upper Amazonia by the Andean uplift. The specific 

 identity of many birds common to both areas is evidence that but little 

 change has taken place in their surroundings since their ranges were dis- 

 connected, and in such cases evolution has, so to speak, been at a standstill. 

 But the elevation of the intervening territory to snow-line has brought into 

 play most of the environmental influences one finds between the equator 

 and the poles, and where in an unchanged basal zone species remained as 

 constant as their habitat, in the new region they sprang forward in an 

 evolutionary race. The evidence on which this theory of the Amazonian 

 origin of Pacific coast life is based is presented in detail beyond. The 

 strongly marked characteristics of the Colombian Pacific Fauna, however, 

 indicate that even in the Tropical Zone evolutionary influences have been 

 active since the isolation of the Pacific coast region. 



The bird-life of the Cauca Valley and upper Magdalena Valley appears 

 to have been acquired under existing topographic conditions. The fact 

 that the forests of the Pacific coast compare in luxuriance with those of 

 upper Amazonia, while forests are of small extent in the Cauca Valley and 

 are wanting in the upper Magdalena Valley, may in part explain the marked 

 difference in the bird-life of these valleys and that of the Pacific coast. 



Heavy forest, however, exists in the lower Cauca-Magdalena region, 

 the bird-life of which has evidently been acquired in part from the Pacific 

 coast, in part from east of the Andes, suggesting that this region was not 

 above sea-level prior to the Andean uplift. These, however, are faunal 

 rather than zonal problems, and they will be discussed more fully in the 

 succeeding pages. 



Our studies of the faunal effects of the appearance of the Andes must 

 not be restricted to those changes wrought by the uplift of this system, but 



