1899] ORIGIN OF A USTRALIAN FL OR A 2 7 9 



towards the elimination of xerophilous forms. But it is not certain 

 that the xerophilous vegetation completely disappeared from South 

 Europe and Asia Minor, for it may well be — to cite a few instances 

 only — that some Chenopodiaceae, and species of Hdicliymm (and a fair 

 number of these still survive in countries bordering on the Mediterranean), 

 may actually be descendants from herbaceous members of the xero- 

 philous flora, and when the present distribution of these genera is borne 

 in mind, there is, it is submitted, at least some probability for this 

 view. 



An objector will, of course, ask why it is, if the theory above 

 sketched be true, that we do not now find species of Eucalyptus and 

 Banhsia and Dri/andsa flourishing in deserts north of the equator. 

 These, he will remark, are precisely the places to which a xerophilous 

 flora would retire for shelter when driven by stress of climate from its 

 former homes. Undoubtedly it would do so, if the desert then existed, 

 and if no stretch of sea interposed to cut off the retreat of xerophilous 

 species. The available desert country reaches from the Atlas Moun- 

 tains across Arabia into Baluchistan, but from this the Sahara must 

 be deducted, since it was, till quite recent times, submerged beneath 

 the sea, and until the nummulitic limestone emerged from the waves, 

 the ocean in which that extensive formation was laid down would be 

 an effectual barrier to migration. Since Eocene times, however, this 

 barrier has not existed ; but it is not clear that Arabia and the drought- 

 stricken regions bordering on it were deserts at the time when the two 

 floras, xerophilous and hygrophilous, were engaged in their life-and- 

 death struggle. If Perim can be taken as a guide — and there is no 

 reason why it should not be- — there is every reason to believe that 

 Arabia enjoyed, in Eocene times, a climate much like that of Europe ; 

 and all we have to suppose is that the same change went on there as 

 in Europe, namely that the climate became more favourable to hygro- 

 philous forms, which were thus enabled to eliminate their xerophilous 

 competitors, and that desert conditions subsequently prevailed, and the 

 absence of Australian genera from the great northern deserts is 

 explained. That this explanation presents difficulties is not to be 

 denied, for the elimination, at least of arborescent forms, has been so 

 complete, we should have expected that at least some few forms would 

 have been able to adapt themselves to the altered conditions. 1 Still 

 the disappearance of these forms is scarcely more remarkable than is 

 the disappearance of " Northern " genera such as Quercus and Alnus 

 and Salix from a country which must have afforded them, one would 

 imagine, many eligible stations where they should have been able to 

 survive. 



Let us now turn to Australia. And first, one must express a 



1 The Indo- Malayan and East Asian species of the Proteaceous genus Helicia may 

 perhaps be cases of adaptation in the sense used above ; so, too, East Asian species of such 

 genera as Drimys, Baeckia, Lcptospermum, Leucojiogon, etc. 



