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



in this interesting group is given at the end of this section. In their distri- 

 bution I believe that we have a clue to the origin of many species of the 

 humid Ecuador-Colombian littoral whose presence in that region may not be 

 otherwise accounted for. Briefly, these forms appear to have been derived 

 from upper Amazonia before the Andes had acquired a sufiicient altitude 

 effectually to separate, as they do now, the Tropical Zones at their eastern 

 and western bases. 



A strong aiBnity exists also between the fresh-water fishes of these two 

 regions. Scharf remarks : " The fresh-water fish fauna of the Pacific slopes 

 of southern Ecuador still exhibits such affinity to that of the Amazon, that 

 the Ecuador mountains could only have had a slight elevation imtU com- 

 paratively recent geological times." ' 



Henn, in confirmation of these statements, writes that " the fishes of the 

 Pacific slope are in general widely distributed Amazonian types; none of 

 them would cause surprise if taken at Manaos." ^ 



Wolf states that the flora of humid western Ecuador is essentially like 

 that of Panama and the Choco region of western Colombia, and adds many 

 species are identical with or belong to the same genus as those found on the 

 eastern slopes of the Andes. (Geographia y Geologia del Ecuador p. 439.) 



Having in mind the possibility of the Amazonian origin of the Pacific 

 humid fauna, W. B. Richardson in 1913, after his explorations for the 

 American Museum on the Ecuador coast, made, at oiu- request, a section 

 across the Andes from Santa Rosa, south of Guayaquil, through Zaruma 

 and Loja to Zamora in the Tropical Zone at the eastern base of the Andes. 

 It is proposed to report on his collections and notes in a subsequent paper, 

 but in this connection it may be said that the results of his journey strongly 

 suggest, as the topography of the region indicates, that this section was one 

 of the latest to be closed to the passage of Tropical Zone forms from one 

 side of the Andes to the other. 



In journeying from Loja to Zamora, Richardson crossed the intervening 

 mountains, which here attain an altitude of 11,500 feet; but the Rio Zamora, 

 rising in the Loja Valley breaks through these mountains at a much lower 

 elevation, below of course, that of Loja, which is given by Richardson as 

 7260 feet. 



This theory of the transandean origin of the Pacific humid fauna affords 

 a satisfactory explanation for the presence in western Ecuador and south- 

 western Colombia of a number of common species which are also represented 

 in eastern Panama, or the Cauca-Magdalena district, but are unknown on 



1 Distribution and Origin of Life in America, 1912, p. 

 ' Arthur Henn, Science, N. S., XL., 1914, p. 603. 



