EVOLUTION 



45 



Fig. 6 

 Breeding distribution of a group of closely-related gulls: Larus occidentalis , 

 the western gull: L. glancescens, the glaucous- winged gull; L. schistisagus , the 

 slaty-backed gull; L. hyperboreus, the glaucous gull; and L. marinus, the great 

 black-back. Areas of overlap shaded. Black areas in Canadian Arctic 

 represent outpost breeding-places of L. marinus 



birds, notably the common guillemot, the three smaller skuas and the 

 fulmar, are polymorphic or dimorphic. They exist in several so-called 

 phases. Some common guillemots have a white ring embracing their 

 eye from which a white line runs back towards the back of their heads. 

 These are called ^bridled' guillemots, and were for long actually 

 thought to be of a different species. The phases of the skuas range 



