1 
THE BIEDS OF AUSTRALIA. 
I here propose to distinguish this form subgenericaUy as 
Ictiniastur, subgen. nov. 
with type Milvus sphenurus VieiUot. 
I do this as thereby we have a name to indicate a most interesting 
episode in bird evolution and zoogeography. It seems possible to trace the 
history of these two birds thus : Ictiniastur in its adult plumage shows the 
immature phase of Haliastur, and we may deduce that it arrived in Australia 
from the north when the birds used that plumage ; when this occurred it 
ranged over Australia and even penetrated into the interior, but before it 
arrived in the South, Tasmania was separated, as it does not occur in that 
• Island. It altered its habits and became accustomed to land life in every 
way and also retained its immature plumage, but developed a little in size. 
Its ancestor stayed in India, kept to the seashore and developed a beautiful 
new adult plumage: then, while bearing the new plumage, it commenced a 
southward travel, and working through the Malay States and the Islands 
arrived in Australia and penetrated down to New South Wales on the east 
and ranged down to West Australia on the west. It also ranged north to 
the Philippine Islands. In each case after it left India it continued its colour- 
change, arriving at its purest stage in Australia and now approaching it at 
the Philippines. In the meanwhile the success of Ictiniastur in adapting 
itself to new conditions led it to range northwards again, and it has reached 
New Guinea and New Caledonia. 
This story may be written in another way, but stiU the theory is worthy 
of consideration, and names should be used for the easy transmission of such 
theory. 
It is worthy of note that these two birds seem to indicate two distinct 
migrations into Australia from the north, both of which took place after the 
breaking of the Bassian Isthmus : by after, is intended to convey the fact 
that the Bassian Isthmus was severed before the first immigration had reached 
that far. The second immigration, as shown by Haliastur, is the one which 
is most evident from a superficial study of the Australian Avifauna. A 
certain companion of Haliastur would seem to be Baza, while the majority 
of the other Australian Falconiformes would seem to belong to the 
Ictiniastur migration. 
146 
