1 91 8,] Platycercine Parrots of Australia. 121 



I have considered these species as a whole, and now 

 suggest the probable dispersion and evolution of the forms. 



I propose to allow a northern origin for the group, and I 

 conclude that the ancestral form was purely green. In the 

 germ-plasm we may assume there was a yellow element, a 

 blue element, and a red element. 



The blue element shows itself in the blue cheeks, blue 

 shoulder-patch, and generally blue tail. The yellow element 

 is seen in the two northern species in full play, subdued in the 

 eastern species, and almost entirely missing in the western. 



The red element is noticeable in all save the two northern 

 species, where, however, it crops up erratically in the north- 

 western form. 



The varying dominance of these elements has constituted 

 the species, but the exact stresses are at present unknown, 

 and the excitation of interest in the search of these is the 

 purpose of this note. 



Thus the geographical distribution of the species admits 

 of theories of evolution without giving clues to the stresses, 

 while these cannot be suggested from knowledge of their 

 environment. 



First, we may suppose that the green bird entered Aus- 

 tralia by way of Cape York, and travelled down the east 

 coast into Tasmania, and along the south coast into south- 

 west Australia. It is probable that the central and north- 

 western parts of Australia were submerged or separated. 

 Tasmania became isolated, so did south-western Australia, 

 the latter division not remaining permanent as did the 

 former. The stress in both cases was towards the preserva- 

 tion of the green coloration, but the erythristic element 

 shows itself in the red forehead, the xanthochroistic in the 

 head and under surface, the cyanistic in the cheeks and 

 shoulder-patch in the Tasmanian form ; while in the south- 

 west species the xanthochroistic has been suppressed in 

 favour of the erythristic, it in its turn eliminating the 

 cyanistic in the cheeks. We thus see a yellow-cheeked 

 species with, however, tlie blue shoulder-patch, while the 

 yellowish under surface has become dull red. Since this 

 stage was achieved a further development seems to be 



