xliv INTRODUCTION. 
point Dr. Schweinfurth and Dr. Bayley Balfour’s botanical investigation of the island 
of Socotra furnishes conclusive data, collected by the latter *. 
About one third of the species of flowering plants of Socotra are endemic; and a 
third are species belonging to tropical Africa and tropical Asia. Exclusive of species 
having a wider area, about eighty-five of the species of dicotyledons are common to 
Africa and Asia, though comparatively few of them penetrate India eastward of 
Scindh. The affinities of the entire Flora of Socotra are essentially tropical African 
and tropical Asian, but the African element predominates, and is mainly composed 
of the features of the Flora of the mountainous region of Abyssinia, with an in- 
fusion of West-African, South-African, and Madagascar types. Among South-African 
types are Graderia, Babiana, Thamnosma, Lasiocorys, and Euryops, of which the first 
two are not known to be represented in the intervening country. In this connection 
it may be mentioned that Dr. Aitchison discovered in Afghanistan the very distinct 
Fingerhuthia, a genus of grasses previously known only from South Africa. It was 
one of the most abundant grasses between Thal and Shinak in the lower Kurram 
valley, and presents no obvious characters to separate it specifically from F. africana, 
though Boissier has described it as a different species f. 
As in the Madagascar flora, so in the Socotran, there are a very few isolated types 
whose nearest allies are in the New World. Balfour specially notices his new mono- 
typic genera Dirachma (Geraniacese) and Celocarpus (Verbenaces) as belonging to this 
category. The three or four Turneracee in Madagascar, the arboreous Mathurina of 
the same order in Rodriguez, and Ravenala madagascariensis are other examples ; 
yet this element is by no means so prominent in the flora as it would appear to 
be in the fauna, judging from Wallace’s remarks thereon f. 
To include the whole of tropical Africa in one subregion is unusual, but the facts 
seem to warrant this course, and the next division should be into several provinces. 
Interruptions in the continuity of the vegetation there are, and the forests of the eastern 
side of the continent are probably nowhere so rich as those of Guinea: but their 
composition is essentially the same. Taking the first volume of Oliver’s ‘Flora of 
Tropical Africa,’ which is, of course, exceedingly fragmentary, it would appear that 
about one fifth of the species there enumerated are common to both sides of the con- 
tinent ; but subsequent investigations leave no doubt that the proportion is really much 
higher. Engler) has analyzed the composition of the flora of tropical Africa as far as 
published in the work cited, namely the Polypetale and the Gamopetale to the end of 
the Ebenacee; but the results can only be used in respect to the general relationships. 
The Leguminose rank first and the Composite next in regard to number of species, 
* Proceedings of the Royal Institution of Great Britain, 1883, and Transactions of the Royal Society of 
Edinburgh, vol. xxxi. 
t ‘Flora Orientalis,’ v. p. 569. 
+ ‘Island Life,’ p. 420. § Versuch, ii. p. 276. 
