xvn AFZELIA BIJUGA 175 



does not note them amongst the seeds stranded on the shores 

 of Krakatoa. They did not occur amongst my collections from 

 the beaches of Keeling Atoll or of the south coast of Java ; nor 

 does Schimper mention them amongst the drift of the Java Sea. In 

 the Botany of the " Challenger " Expedition the species is not even 

 referred to in any connection. Although, however, the capacity of 

 these seeds for dispersal by currents is for the first time established 

 by me, their fitness in this respect was surmised by Schimper 

 (p. 191), when he placed the species in his list of tropical shore 

 plants evidently distributed by the currents. 



It will thus be gathered that we have yet much to learn in this 

 matter ; and I would recommend any resident in the tropics to 

 take up this subject. When indeed we remember the fine adjust- 

 ment existing between the specific weight of the seeds and the 

 density of water, and recall the unknown factor determining the 

 difference in buoyancy between the kernels of coast and inland 

 seeds, we can understand how under particular conditions in certain 

 portions of its range the seeds of Afzelia bijuga may perhaps never 

 possess any floating power. It would seem, in fact, that the seeds 

 are much more buoyant in the Western Pacific than they are in the 

 Java Sea ; or it may be that the tree is much less frequent ; or that 

 the stranded seeds are soon destroyed by crabs, such as is the fate 

 of much seed-drift on the Keeling beaches ; or lastly that, as in 

 Diego Garcia, rats in destroying the fallen seeds are bringing about 

 the extermination of the species. 



Summary relating to Afzelia bijuga. 



(1) Assuming that the genus has its home in the African 

 continent, and that the species have frequently a riverside station, 

 it is argued that the distribution of the genus on both sides of that 

 continent can only be explained by its dispersal by rivers from a 

 centre in the interior. 



(2) Afzelia bijuga, a widely distributed shore tree of tropical 

 Asia, occurs in Fiji both at the coast and in the inland forests. 



(3) This double station is associated inter alia with a 

 different buoyant behaviour of the seeds, those of the coast trees 

 usually floating for long periods, whilst those from inland generally 

 sink. 



(4) There can be no doubt that this widely ranging littoral tree 

 has been dispersed by the currents ; but the specific weight of the 



