282 SPENCER MOORE [october 



the two areas. But the evidence from species is not a satisfactory- 

 disproof of communication, for desert varieties are, as is well known, 

 rather frequent, and if in the course of ages species have been 

 differentiated from such varieties, a fair amount of concealed inter- 

 change may have taken place. The evidence is much stronger in 

 respect of genera restricted either to the east or the west side of the 

 country. Of these there are a large number which have not succeeded 

 in making the passage, although many — xerophilous ones especially 

 — have advanced some way towards doing so. These cases give 

 emphasis to the conclusion that interchange across the desert has 

 taken place very slowly, and to no considerable extent on the whole, 

 although I cannot help thinking that Professor Tate decidedly under- 

 estimates its amount. 



For long periods, perhaps since Cretaceous times, the evolution of 

 the . flora of Eastern and Western Australia has proceeded along 

 different lines. So far I am in accord with other writers, and indeed 

 this seems the only inference to be drawn from the facts. But the 

 main reason for this is to be sought, as I venture to think, not in the 

 simple isolation of the western part of the country while the eastern 

 has been accessible to migrants from outside which have made 

 headway against the endemic vegetation in consequence of their 

 inherent superiority, but in climatic difference which had already 

 become pronounced while the eastern interior was still a comparatively 

 well -watered country. In short, I see Western Australia to-day 

 supporting a vegetation similar, in its chief elements, to that which 

 would now have been flourishing in Europe if our continent had been 

 undergoing desiccation since Miocene times, and without lowering of 

 its mean annual temperature. The interposition between the two 

 halves of Australia of a sea and of a desert has, no doubt, laid an 

 embargo on migration from one to the other. But for these barriers 

 many restricteclly eastern forms would now be found upon the 

 western seaboard, and vice versa. But we are not warranted in 

 supposing that interchange would have taken place to such an extent 

 as to result in a homogeneous flora ; for the areas in Western Australia 

 suited to hygrophilous forms are strictly limited, and the pre- 

 ponderating xerophilous element in the western flora is so well 

 adapted to the extraordinary conditions prevailing in the west as to 

 render its displacement in the highest degree unlikely. 



Turning now to Eastern Australia, we find there a flora with little 

 ordinal difference from that of the west, but containing many genera 

 and a large number of species which, if they advance westward into 

 the desert at all, do not reach the coast. Moreover, speaking 

 generally, as we proceed northwards, and this applies generally to the 

 moister regions near the coast, the number of forms possessed in 

 common with Indo-Malaya and of forms allied to such tends to increase. 

 There is also a sprinkling of species now characteristic of northern 



