CHAPTER IV 



THE SIMILARITY BETWEEN THE WEST INDIAN AND WEST AFRICAN 

 LITTORAL FLORAS AS EXPLAINED BY CURRENTS 



The similarity between the West Indian and West African littoral 

 floras has long occupied the attention of botanists, and particularly 

 of the late Prof. Schimper. It is a subject that is intimately bound 

 up with the question of the dispersal of plants by currents ; but a 

 little clearing of the ground is first requisite in order to appreciate 

 its true significance. 



The Discontinuity of Tropical Genera possessing Littoral 

 Species. — A very noticeable and not infrequent feature in connec- 

 tion with the littoral trees of the tropics of the Old and New World 

 is the discontinuous distribution of the genera as far as all the non- 

 littoral species are concerned. Though the littoral species is in each 

 case widely distributed in both the western and eastern hemispheres, 

 the genus is also represented by peculiar species in both worlds, and 

 in such a manner that we are often left in doubt whether the shore 

 species has its home in the east or in the west. In all such cases 

 it is only the littoral species spread by the currents that connects 

 the two hemispheres. Otherwise the discontinuity would be com- 

 plete. As examples we may give Chrysobalanus, Entada, Thespesia, 

 and Ximenia. But this discontinuous distribution raises many 

 difficulties, whilst it removes others. 



Thus, as far as the range of the genus is concerned, the New World 

 has as much claim as the Old World to be considered the home of 

 Thespesia populnea, since the six other known species are shared 

 equally by the two hemispheres. But other genera than those 

 named above present new points of difficulty. Thus Symphonia 

 is a genus of twelve or thirteen species, of which nearly all are con- 

 fined to Madagascar and two occur in Africa. A single estuarine 

 species, S. globulifera, which connects West Africa with the New World 

 has apparently no means of crossing the Atlantic. Then we have 

 Terminalia with a large number of species spread over the tropics 

 of both hemispheres ; but truly littoral species occur only in the Old 

 World, and there is no species connecting the two hemispheres. 

 Then there is Crudya with nearly all its dozen species American. 



This discontinuity presents no great difficulties when it affects 

 genera found in the tropics of both hemispheres which are only 

 connected together by littoral species that are known to be dispersed 



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