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 restrictedly 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 ina 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 

