ON THE INFLUENCE OP PHYSIOGRAPHIC CHANGES. 321 



Whatever may have been the cause, which brought about these 

 very opposite climatic conditions, the facts are incontestable and 

 point to one conclusion, that since Pliocene times Central Aus- 

 tralia has been drying up — that the present barrier to the 

 migration of plants has simply replaced one ditierent in kind — an 

 arid waste by a vast area of inland drainage. 



As the aridity of Central Australia is a consequence of its 

 geographical position in reference to tlie equatorial region of low 

 barometric pressure, being the co-ordinate region of high barometric 

 pressure, it is not illogical to suggest that a secondary cause of 

 the pluvial conditions in Pliocene times over Central Australia 

 was the northerly sliift of the whole system of winds, by which 

 the dry region moved to the north, and efiectually acted there as a 

 barrier to migration, whilst in more southern latitudes the inland 

 sea operated in a like manner. 



The oscillation of climate through long periods of time as 

 e\dnced by Horal features, has been generally accepted for the 

 Northern Hemisphere, and it is not unreasonable to insist that 

 corresponding climatic vicissitudes had been experienced in this 

 • hemisphere ; but on the hypothesis of Dr. Croll, the maxima and 

 minima would alternate in the two hemispheres. The temperate 

 conditions which prevailed over the arctic regions in Mid-Tertiary 

 times, and clothed them in a forest vegetation, had come to be 

 replaced in the Glacial Epoch by the exti'emest rigour of a long- 

 continued arctic winter, which involved the present cold temperate 

 belt of the old world in a high condition of refrigeration ; since 

 then the climate has ameliorated. An analagous sequence of 

 phenomena, diftering in intensity but not in kind, has been 

 witnessed in these latitudes ; the northerly advance of the region 

 of high barometric pressure brought the now arid region into the 

 belt of summer rains, whilst in more southern latitudes a mild 

 glacial epoch prevailed. 



Thus, whilst the Eremian Region has been a land surface since 

 Cretaceous times, it has been subjected to sucli varying climatic 

 and physical changes that must have materially atiected the 

 permanency of its plants. We cannot wonder, therefore, that in 

 coniparison with the Autochthonian and Euronotian floras, its 

 plants should present less of an Australian facies than its neigh- 

 bour, consisting virtually of emigrants from surrounding provinces, 

 though suflicient time has elapsed to have developed within itself 

 some new generic types and evolved many new species. 



The more modern facies of the Eremian floi'a is partly shown 

 by the wide disposal of a large number of exotic species, wliich 

 thus evince a quality of accommodating themselves to a great 

 variety of climatic and physical influences. This colonising power 

 has had freer scope to exercise itself in the comparatively pre- 

 occupied tract of the Eremian Region, than was possible in the 

 more stable external regions. 



V 



