558 DEVELOPMENT OF THE N.O. MYRTACEjE, 



This group forms another great, bifurcating limb springing 

 from a point higher up the stem of the genealogical tree of the 

 Eucalypts, than that which gave rise to the Boxes and the Iron- 

 barks; and their extreme types, such as E. microcorys, E. stel- 

 lulata, E. coriacea, E. cocci/era, and E. hcema stoma are as far 

 removed from the general succession as are the extreme types of 

 the Boxes and Ironbarks. 



In the more arid portions of the continent, lying between these 

 various groups, the desert-types were developed in the fulness of 

 time. Types, E. oleosa, E. dumosa, E. incrassata, E. uncinata, E 

 gracilis, and E. pachyphylla. 



In brief, the history of Eucalyptus appears to have been as fol- 

 lows : A prototype of the Metrosiderese, in the late Cretaceous, 

 secreted a pinene oil, in Northern Australia or the neighbouring 

 regions, and succeeded in forming an operculum by the coherence 

 of its petals, for the protection of its reproductive organs. The 

 earlier types appear to have possessed parallel anthers opening in 

 longitudinal slits, stamens often brightly coloured, large glossy 

 leaves, with transverse venation, a thick bark, pinene oil ; and they 

 appear to have grown in a porous sandy soil, and in a warm to hot 

 climate. Moreover, the earlier leaves were opposite, sessile, and 

 cordate, with horizontal surfaces. Protected by the operculum 

 and the essential oil, the new plant began to push its way into 

 the cooler country to the south. In proportion to such progress, 

 and to its adaptation to a porous soil, it lost its juvenile opposite 

 leaves. 



With the retreat of the Cretaceous Sea, the Eastern side of Aus- 

 tralia underwent a geological and geographical transformation, 

 and, in their attempts to respond to their peculiar environment, the 

 Eucalypts secreted a non-volatile wax-bloom, and two fresh oils, 

 cineol and phellandrene. The function of the oils appears to be, 

 in the main, to form a thin spray to withstand desiccation, but 

 also, in the case of phellandrene and certain other constituents, 

 their function appears to have been in part, at least, to resist cold, 

 because the Metrosidereae, with the Myrtese, are unable to resist 

 cold equally with aridity. The twisting of the leaf -stalk, and the 



