Family and Generic Differentiation. 329 



Caciques or Oropendolas of the family Icteridae*. In 

 this series we have represented five genera, and, from the 

 point of view of the structural features of the bills and 

 other characters, I think it will be allowed that they are 

 five genera as well-marked and distinct as any that could 

 be found in any one group. Yet, as you will notice, the 

 colour-pattern is fixed and absolutely characteristic. 



There is another point which I must now briefly notice : 



(4J The correlation of colour-pattern with geographical or 

 faunal areas. 



This seems to me an important side of the question, as 

 not only showing that colour-pattern is a deep-seated 

 phenomenon, but as pointing to the very secondary, if 

 any, part which mere environment has played in its 

 determination. 



I propose to take the genus Ccereba as an example. 

 Those members who are familiar Avitli South American 

 birds will remember that this genus of brightly-coloured 

 Honey-Creepers can be divided into two groups, namely, a 

 group with white spots on the wings, and a group without 

 white wing-spots. 



Here is a map illustrating the distribution of these tAvo 

 races f — the white wing-spotted race being represented in 

 green, and the race without wing-spots in red. 



Now, the two areas respectively occupied by these two 

 races of Coereba are, although now connected throughout 

 the greater part of their extent, absolutely distinct in origin, 

 and present entirely different geological features and history. 

 The area coloured red, for instance, represents the extremely 

 ancient and, at one time, insulated Brazilian and Guianan 

 land-masses. It consists of the most ancient rock-systems 

 known — Pre-Cambrian gneisses and Archean schists. On 

 the other hand, the area coloured green consists almost 

 entirely of Tertiary deposits which have been comparatively 



* Examples of the following genera were exhibited, viz., Clypeicterus, 

 Eucorystes, Gymnostinops, Ostinojfs, and Ocyalus. 



t Cf. also'Ibis,' July 1912. 

 SER. X. VOL. 111. Z 



