^°'i9™] CAMPP,ELL, .\. G., Acanthhac or Tlionibills. 193 



south of Cape Jveeuuin and that portion was joined to the east- 

 ern extremity of Austraha, whicli tlien included Tasmania as 

 part of the mainland. 



In the study of geolo.s:y the vast changes that may occur are 

 an im])ressive lesson, and time only was required for the Great 

 Australian Hii^ht to ai)pear and work its way, assisted, no doubt, 

 by various earth moxements, acrf)ss the timbered belt of good 

 conditions. 



In the south-west there was no indigenous Kookaburra or 

 Lyrebird, to name two well-known species. They must have 

 arrived in the south-east after the appearance of the barrier. 

 On the other hand there are certain passerine birds which show 

 an intimate connection, east with west, namely, Pachycephala 

 and Atrichornis, and in particular .Icanthiace. It is reasonable 

 to assume that these were well established in east and west 

 before the break occurred. Pachycephala shows little or no 

 alteration western to eastern form. Atrichornis, from its strictly 

 limited habits, shows a very marked difference both in size and 

 coloration, while the /Icanthiccr are at a very interesting stage 

 of evolution; eastern and western forms have become sufficiently 

 differentiated to warrant specific rank, while new races or sub- 

 species are developing in conditions that are considerably drier 

 than those in which the original stock lived. 



Another geological fact we must notice at the outset. In 

 Tertiary times a great portion of southern Australia was sub- 

 merged, namely, a great basin extending from about Portland 

 in Victoria, eastward to near Corowa on the Murray, round in 

 a great semi-circle to Broken Hill and down near Adelaide. 

 Another great area extended from Spencer Gulf well inland 

 around the head of the Australian Bight and back to the coast 

 near Esperance Bay. 



This subsidence must have had a most marked effect on the 

 distribution of bird-life generally, and it will be noticed as we 

 proceed with examination of the AcantJiizoo that it is an important 

 factor in the distribution of that genus. All the eastern and 

 southern highlands, the Mt. Lofty Ranges, the country north from 

 Tarcoola to McDonnell Ranges, and most of West Australia, 

 stood out around those Tertiary seas and probably the forests of 

 those parts and their bird inhabitants were not very different 

 from each other. They were the immediate ancestors of all our 

 present species, and perhaps more which have passed out of 

 existence. 



When the Tertiary seas retreated and left exposed those bound- 

 less plains, what a wealth of new country awaited population! 

 On account of a low rainfall the central area at the head of the 

 Bight is now a "desert," and a very substantial barrier to further 

 east-west movement of bird-life. That part between the coast- 

 line and Musgrave Ranges is the most desert-like tract inhabited 

 \>y AcaniJiiaa:. 



