228 I The Process of Evolution 



beak and then probing the excavations with a cactus spine or twig. 

 This remarkable behavior compensates for the lack of a long tongue 

 to use as a probe and is one of the few instances of birds using a 

 "tool." On the only island occupied by both C. pallida and C. helio- 

 bates (Albemarle Island) their ecological differences isolate them 

 (C. heliobates, in the coastal mangrove belt; C. pallida, inland). 



Fig. 1 0.8 I Extremes of ditierentiation in skulls of Galapagos finches. 

 Upper, Geospiza magnirostris ; lower, Certhidea olivacea. {After Bowman, 

 1961, University of California Publications in Zoology 58.) 



