Geog rap] ileal Dif<tribut!on of Sf'a-Grasses. IS-") 



south coast (how far \vest\Yards 1 do not know); also in New Zea- 

 land and in one place on the coast of Chili. Thus the area covers 

 the temperate part of the South-Pacific, with an outpost in the 

 Tropics at Cape York. 



The discussion of the orifi:in of this part of the Australian group 

 will be postponed and taken together with that of other species 

 of Zostera. 



The other section of the Australian group consists of two species. 

 The first — Cymodncea anforcfica — stands somewhat isolated with- 

 in the genus. It is quite different from its nearest ally, the East- 

 African C ciliofa, and it must be supposed to be an old species. 

 The other — Fosidonm ausfralls — has only one congeneric species, 

 P. oceanicn, of the Mediterranean. Thus, these tAvo species, which 

 make up the isolated genus Posidonia, inhabit widely separated 

 and comparatively small areas of a Avarm-temperate charactei-. 



The species are quite distinguishable from one anothei', in good 

 agreement with their I'emote areas of occurrence. 



The marked specific differences, as Avell as the isolated place of 

 the genus within the Potamogetonaceae, indicate their" great age. 

 The following more detailed explanation of their distribution is 

 only a Avorking hypothesis. In former times the genus inhabited 

 & continuous area, of Avhich the present tAvo isolated areas are the 

 only remnants. It seems as if the genus is noAV no longer fit for 

 true tropical conditions, while the ancestors of the present species 

 did occur in the Tropics. Tropical conditions, tlien, have driven 

 Posidonia toAvards the north and south; and the Australian south- 

 coast and the Mediterranean are the last refuge for a dying genus, 

 Airhich, to judge from identifications of leaves from Tertiary -.'and 

 Cretaceous times, is one of the oldest floAvei'ing plants. 



VI. The Mediterranean group contains only tAvo species — viz. : 



Cymodocea nodosa. 



Posidonia oceanica. 



The tAVo species, Zostera marina and Z. nana also uceur in the 

 Mediterranean, but are not lestvicted to it. 



We have already dealt Avith I'osldonla oceanica. Avhen treating 

 of the Australian /'. australis. Neither Posidonia nor Cymodocea 

 goes into the Black Sea, probably because its waters are neither 

 Avarm nor saline enough; Zostera marina and Z. nana, on the 

 other liand, having greater ability of adaptation, penetrate into 

 the Black Sea. 



