THE BIRDS OF AUSTRALIA. 
a 111 longirostris. 
b 1 ( hypoleuca ) hypoleuca. 
b 11 leuconota. 
b 111 dorsalis. 
This arrangement is not much different from the generally accepted ones, 
the status of the groups A and B being doubtful and their specific value is 
not invalidated by either Hall’s or Coles’ observations. 
I would not hypothesize a “wholly black” ancestor as this is absolutely 
negatived by the immature plumage stages, a mottled greyish bird being 
suggested ; then the evolution of a white-backed from a black-backed form 
may be admitted, but apparently it has been a semi-independent evolution; 
probably the pure black-back evolving in the north at the same time as the 
pure white has developed in the south from a mottled blackish-back form. 
It is possible that the white-backed form is at present more virile and 
may eliminate the black-backed forms wherever they meet as has been done 
entirely in Tasmania and South-west Australia. The geographical distribution 
of the two species is worth a little notice. In the extreme north of Queensland 
and apparently Melville Island and some parts of North-west Australia it 
is at present absent, but it has reached Groote Eylandt in the Gulf of 
Carpentaria. I have pointed out that the New South Wales form of G. 
hypoleuca is the largest, while G. tibicen also reaches its largest size in that 
State. The northern forms of G. tibicen are the smallest and the southern 
forms of G. leuconota are the smallest. 
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