According to Bentham the predominant part of the Australian vegetation appears as strictly indigenous and the endemic species and genera must have originated or differentiated in Australia without ever leaving it. There are few exceptions, such as certain species of Eucalyptus, Epacridaceae and Phillodinus Acacias found in Malaysia and few annuals or herbaceous plants that have been found in southern China.
“The most singular phenomenon, represented by this wonderful Australian endemic flora, says Andrews (I, p. 231), is the affluence, segregation or aggregation of all possible types of Phanerogams on sterile uninviting soils and sandy during the great climatic differentiations of the lower and post-Cretaceous period “. Mirtaceae, Rutaceae, Proteacee, Sterculiacee, Euphorbiacee left the jungle to form new genera and species on arenaceous soils; orchids that had been epiphytes in the jungle formed new terrestrial genera; Epacridaceae, Verbenaceae, Labiate and Umbrelliferae, having become dwarf, emigrated towards sterile and sandy places.
According to TOPPHARMACYSCHOOLS, this secondary flora reached a high degree of specialization. Numerous means were developed to reduce transpiration, such as the sinking of stomata, the great development of woody tissues, the reduction of intercellular spaces, the secretion of wax, resin and volatile oils, the reduction of the surface of the leaves. This vegetation appears to have originated in the temperate regions of the northern hemisphere and partially in the tropics. From these took place the migration towards the southern part in Africa and Australia, where similar climatic and edaphic conditions led to the formation of similar floras, and in South America, where totally different conditions were encountered. In the Cretaceous period and for most of the Tertiary there was no it was an Australian continent as we know it today. Wallace, in his Island Life, says that today’s vegetation in Australia has most differentiated in the southwestern part of the continent. Australia consisted of a large western island almost entirely extratropical and a long, narrow eastern island that stretched from Cape York to beyond Tasmania, separated by a wide sea with small islands. In these epochs, southern and western Australia was the largest part of the continent, and a robust xerophilic flora was able to differentiate in its sandy and sterile soils. The disappearance of the Cretaceous sea made possible the migration of this western flora towards the east: the xerophilous plants became so invasive and vigorous, that their diffusion was later arrested only by the isolation of the continent and by the
Currently the Australian flora shows some interesting affinities with other flora. A number of genera of tropical Asia extend into tropical and subtropical eastern Australia; sometimes with identical species, other times with more or less different species. Coastal Queensland plants have Asian-Oriental characters. The Austro-Malay element is common in the eastern forests of Queensland and New South Wales, and extends along the east coast, as far as humidity, sheltered conditions and soil conditions are favorable. There is also a certain affinity with the flora of New Guinea, although not as great as one might expect. The savannas of the Fly River region in New Guinea are nearly identical to those of the Cape York Peninsula in Australia. The herbs of these savannas are Imperata arundinacea, Themeda, Andropogon, and with them are various species of Eucalyptus (E. Alba, E. Tereticornis, E. Clavigera, E. terminalis) and other Mirtacee, some species of acacia (A. Simsii, Australia Notosericea) and some Proteaceae.
The finding of epiphytic rhododendrons at great heights in New Guinea allows us to reconnect this alpine flora with that of the Himālaya: Rhododendron has also been found on the Bellenden Ker Range in north-eastern Queensland. This mountainous relief is interesting as species of Agapetes and Dracophyllum have also been found there. There are 30 species of Agapetes collected from the Himālaya and a smaller number from Borneo, Fiji and New Guinea. Dracophyllum instead it represents an Antarctic element, and a common genus in the austral islands and in Tasmania. One species is also found in New South Wales, Lord Howe Island, New Zealand and New Caledonia, especially in the mountainous parts. The alpine flora of Tasmania, Victoria and New South Wales show many similarities and may be connected with those of the southern temperate mountainous region that stretches across New Zealand to the southern tip of America and goes up from there to the Andes..
Although there is a remarkable similarity between the flora of southern Africa and that of Australia, nevertheless there are few genera and no species common to the two continents, other than the cosmopolitan plant species which are not limited to these two regions.
Some biologists argue that at the end of the Cretaceous period there must have been a land bridge between southern Africa and south-eastern Australia: this is based on the presence in both countries of Composite genera such as Helipterum, Helichrysum and Cassinia, of Restiacee like Restis, Hypolaena and Leptocarpus, of Liliacee like Caesia, Bulbine and Wurmbea and of the Proteae and Persoonieae tribesof the Proteaceae, while the Ericaceae family is well represented in South Africa and the related Epacridaceae are found above all in extra-tropical Australia. But, given that such a land bridge had existed, it seems strange that the numerous Ericaceae, of which Erica alone contains 470 species in southern Africa, did not benefit from it, and that the large and intrusive genera of the Australian Epacridaceae did not find for it their way to southern Africa. Also the absolute absence in this of the genus Eucalyptus and of every species of the invasive Leptospermeae and Chamaelaucieae tribeswhich grow alongside Proteaceae and Epacridaceae in Australia, suggests that there was no such bridge. This fact indicates rather that the Proteaceae and other families common to both countries thrived in the northern hemisphere and were pushed south into Africa and Australia, where, isolated and free from competition, they developed new tribes, sub-tribes, genera and species.