As a step towards such hypothesis it has been noted that the Antarctic, the South African, and the Australian floras have many types in common.
To quote further: Existing floras exhibit only one moment in the history of the earths vegetation.
The remark may conveniently find its place here that plants which have reached a high degree of adaptive specialization have come to the end of their tether: a too complicated adjustment has deprived them of the elasticity which would enable them to adapt themselves to any further change in their surroundings, and they would pass away with conditions with which they are too inextricably bound up. Vast floras have doubtless thus found their grave in geologic change.
As Clement Reid remarked: World-wide floras, such as seem to characterize some of the older periods, have ceased to be, and plants are distributed more markedly according to geographical provinces and in climatic zones.
Starkie Gardner has argued with much plausibility that the Tertiary floras which have been found in the far north must have been of Eocene age.