xxvii HAWAIIAN RESIDUAL GENERA 363 



method of dispersal is not easy to discover. Thus, Mezoneuron, a 

 Leguminous genus with seeds an inch across (2*5 cm.), and 

 Peucedanum, of the Umbelliferae, with mericarps half to three- 

 quarters of an inch (1*2 to r8 cm.) in length, offer serious difficulties 

 to the student of plant-dispersal. In discussing the difficulty con- 

 nected with Mezoneuron (see Chapter XV.) he will keep in view the 

 possibility that the original species may have been a littoral plant 

 possessing seeds dispersed by the currents, seeds that lost their 

 buoyancy when the plant established itself inland, just as is now 

 taking place with Afzelia bijuga, a Leguminous littoral tree 

 of Fiji (see Chapter XVIIA 



He will also find much to puzzle him in the mode of dispersal 

 of the Hawaiian residual genera of the Convolvulaceae (Breweria, 

 Jacquemontia, and Cuscuta) that possess only endemic species, 

 and he will speculate as to the manner in which seeds that would 

 seem to possess but little attraction for birds and have no capacity 

 for transportation by the currents could ever have reached these 

 islands, and he will ask himself why it is that the agencies of 

 dispersal, whatever they are, have now ceased to be active. He 

 will perhaps see a way out of his difficulties when he perceives that 

 if isolation has led to the development of peculiar species in 

 Hawaii, it has strangely enough in the case of the Myrsinaceous 

 genus Embelia produced the same effect over the whole range of 

 the genus, and that Hawaii has in this respect derived no advantage 

 from being an oceanic group. According to Carl Mez, nearly all 

 the ninety species of this Old World genus are restricted in their 

 areas, whether continental or insular (" Myrsinaceae," Das Pflanzen- 

 reich, 1902) ; and indeed we do not seem justified in assuming that 

 the isolating influences in the case of this genus have been more 

 effective in Hawaii in the mid-Pacific, or in Mauritius in the Indian 

 Ocean, than they have been in continental regions like the Deccan 

 and Nyassa Land, in all of which localities endemic species occur. 



The remarkable development of the Cucurbitaceous genus 

 Sicyos, in Hawaii alone of all the tropical Pacific groups, will 

 attract his attention, and he will find here another instance of that 

 predominant principle in the distribution of Pacific plants, where 

 in a widely- ranging genus we find one of its species covering most of 

 its area, whilst the other species are more or less localised. He 

 will wonder at the limitation to Hawaii of a genus like Dracaena, 

 that is so well adapted for dispersal over the Pacific by frugivorous 

 birds ; and in endeavouring to explain the presence in the 

 Hawaiian forests of the gigantic Rumex, R. giganteus, he will 



