174 HORN EXPEDITION — SrMMAHY. 



Professor Tato ami Mr. II. Y. L. ]!rown, thorn can bo littlo doiiht but tliat the 

 work of Mr. Wallace in regard to the clisti'ilmtion of animals and jilants in 

 Australia is second in importance to that of no other writer, both in relation to 

 its suggestivenes.s and to the conclu.sions which In; draw.s, though at the .same 

 time, with increasing knowledge, it may lie necessaiy to modify ceit.iin of the 

 lattei-. 



What seems to have been probably the history of the changes during and 

 since Cretaceous times in the centre of the continent is : — 



1. The existence of a great marine area in which the Upper Cretaceou.s rocks 

 forming the IJolling Downs system were deposited, and by which the central 

 district now foi'ming the Higher .Steppes and including the McDonnell, James and 

 other ranges in the Centre was isolated from both the botanical Autochthonian 

 region on the west and the comparatively narrow coastal strip on the east and 

 south-cast. 



2. After the elevation and partial denudation of the Rolling Downs .system 

 another submergence occurred, when the same region was occupied partly by a 

 marine but maiidy and especially in the centi'al-southi'rn an^a by a great Lacustrine 

 area, and at this time the Desert .Sandstone (Supra-Ci'etaceous) was deposited. 



3. The Lacustrine area gradually diminished, but pluvial conditions or at all 

 events a greater rainfall than the present one continued into Pliocene times. 



4. During the latter periods the Coastal IJange, then much higher than now, 

 formed a barrier lietween the lai-ge, internal, well-watered area and the narrow 

 coastal strip. 



5. In Post-Pliocene times desiccation ensued. 



.So far as the flora is concerned the original division of the continent into a 

 western and an eastern half, the former containing the Autochthonian constituent, 

 is generally admitted. At the present time, the former, which, as is generally 

 agreed upon, was isolated from the eastern half during Cretaceou.s times, 

 contains the typical Australian series of genera and is, in this respect, to bo 

 strongly contrasted with the Euronotian or eastern flora which was, as Pi-ofessor 

 Tate s;iys, "superimposed by the Oriental and Andt'an incursions." To these may 

 be added the same author's conclusion that the Eiemian flora was developed in 

 Central Australia in Pliocene times "from Autochthonian and Euronotian elements 

 and lai-gely modified by Oriental inmiigrants." 



