ON THE INFLUENCE OF PHYSIOGRAPHIC CHANGES. 313 



By way of introduction I have to traverse ground that has 

 been substantially prepared, which may be summarised briefly as 

 follows : — 



■^ The Mora of Australia consists of the following constituent 

 elements : — 



I. — An immigrant portion, derived from at least two separate 

 sources. 



(a) Oriental, which is dominant in the littoral tracts of 

 tropical Australia ; but despite the large assemblage of 

 Asiatic species, the Australian character is deeply 

 impressed by numbers, specitically and individually, of 

 £ii(:a/i/ptus, Grevillea, Phyllodineous Acacia, and others. 



(b) Andean (including also certain species of the cool and 

 temporate regions of the Noi'th Hemisphei-e). For the 

 most part this type of vegetation is restricted to the high 

 mountains of Tasmania, Victoria and New South Wales. 

 The peculiarity of the Tasmanian flora is only in its 

 alpine types. 



11. — An endemic portion, a localised type of which occupies 

 the extreme south-west of the continent. 



It is conceded that the large and varied flora of the south-west, 

 specifically, almost totally so, and largely generically dififerent, 

 implies long continued isolation. Much speculation has been 

 indulged in as to the physical causes which haA"e brought about 

 and maintained tliat isolation. The whole question, seems to me, 

 to resolve itself to this : — Is there independent of the pecu- 

 liarities of the distribution of the flora, and incidentally of the 

 fauna, any evidences of alteration in our continental physiogi'aphy 

 which may account for the biological phenomena. I answer, yes ! 



The cliief factors influencing the geographic disti-ibution of 

 plants are those of temperature and moisture, because they are 

 indispensable ; of the two, so far as Australia is concerned the 

 latter is by \ery far the more important. Petrological conditions 

 are of secondary value only, though gi^■ing rise within limited 

 areas to great floral contrasts ; this is exhibited on a rather 

 extensixe scale by the vegetation of tropical South Australia. If 

 we pass from the coast inland in a .southerly direction, a very 

 abrupt change is encountered on gaining the escarpment which 

 terminates the plateau of the Desert Sandstone. In the basin 

 of the rivers of Arnheim Land, we have a retentive soil and 

 copious rainfall contrasting with the highly absorbent soil and 

 diminished rainfall of the Table-land ; it is to the superior hygro- 

 metric property of the soil of the latter, rather than to any 

 chemical properties of the formation, that is due the great differ- 

 ences in the floral productions. In the same region, there occur 



