396 SIB J. B. HOOKER ON THE PLA.NTS OF THE 



have advanced even as far as the Cape Colony, besides many- 

 others that have not gone so far, and of which latter a few have 

 been collected by Dr. Kirk during Dr. Livingstone's second 

 expedition, in the mountains of comparatively low elevation near 

 Lake Shirwa in lat. 15° S. In a lesser degree it has been 

 peopled by a return flow of South- African genera and species, 

 of which many have in like manner advanced further and reached 

 Abyssinia, whilst others have been arrested in their northward 

 spread. It would be interesting, but in the present state of our 

 knowledge fruitless, to speculate on the direction in which the 

 wave of migration is now advancing, and whether the later northern 

 preceded the southern, or vice versa. Yet when it is considered 

 that the whole area over which Mr. Thomson's collections were 

 made is volcanic, and probably geologically modern, in its present 

 configuration, it must be evident that the main features of its 

 vegetation are of no great antiquity. 



There is one more point of interest to which a study of Mr. 

 Thomson's collection invites attention, which is that, whereas the 

 lowlands of Eastern Tropical Africa (and indeed of all Tropical 

 Africa) abound in species and representative species of the Deccan 

 peninsula of India, the highlands of these two regions seem to 

 have nothing in common, botanically or zoologically. And what 

 renders this more noteworthy is that, though they have no types 

 in common, there are desiderata common to both, as exemplified 

 by the absence of Cupuliferse, and paucity of Conifer®, Cycadeae, 

 and Palmeae, all of which abound in the Eastern Archipelago and 

 in most other tropical countries. Looking still further off, and 

 comparing the African flora with the Australian, a singular dif- 

 ference is observable in this, that whereas the Tropical- Australian 

 flora is in very great measure made up of species belonging to 

 Temperate- Australian genera, the Tropical- African is, except in 

 the highlands, of a totally different type from the South- African. 

 The tropical floras of both have been obtained largely from the 

 Asiatic continent ; but whereas in Australia there is a mingling 

 of the Asiatic and endemic southern genera and species, there 

 is no such mingling of the elements in Africa, except at con- 

 siderable elevations, in its tropical regions. 



J. D. Hooker. 



