XV LITTORAL AND INLAND PLANTS' RELATIONSHIP 145 



through the agency of the currents, and the great difficulty in 

 believing that their beans were carried unharmed by birds over 

 half the breadth of the Pacific Ocean. If we reject alike the 

 current, the bird, and the parentage of a lost littoral species, we 

 must fall back on the continental hypothesis, against which in the 

 case of Hawaii the evidence is overwhelming. 



'&• 



Canavalia. 



This genus is represented in the tropical islands of the South 

 Pacific from Fiji to Tahiti by three littoral species, none of which 

 have been found in Hawaii, where only an endemic inland species 

 -exists. Reference will alone be made here to such facts as bear 

 on the probable history of the mysterious Hawaiian species, 

 additional particulars being given in Note 54. The littoral species, 

 Canavalia obtusifolia (D. C), C. sericea (Gray), and C. ensiformis 

 (D. C), have buoyant seeds and are dispersed by the currents ; 

 whilst the inland Hawaiian species, C. galeata (Gaud.), a forest 

 climber peculiar to that group, has non-buoyant seeds. We thus 

 have repeated the problem of Erythrina monosperma. The absence 

 of the littoral species from Hawaii can scarcely be attributed to the 

 failure of the currents, since Ipomea pes caprae, which accompanies 

 C. obtusifolia as a beach-creeper all round the tropical globe, is 

 present on the Hawaiian beaches. Nor can it arise from lack of 

 floating-power on the part of the seeds, since experiment indicates 

 that the seeds of C. obtusifolia will float for months unharmed in 

 sea-water. Nor can it be ascribed to climatic conditions, since 

 this tropical shore species extends into cooler latitudes than 

 those of the Hawaiian Islands, being found in the Kermadec Group 

 and in the Bermudas, which are subtropical both in position and 

 as regards much of their vegetation. The reason perhaps we may 

 never learn from the plants themselves, though it may be possible 

 to obtain some light on the problem from outside sources. 



Canavalia galeata differs much in its habits, as well as in some 

 of its characters, from the existing littoral species of regions outside 

 the Hawaiian Group. It is a stout climber ascending the forest 

 trees to a considerable height, though, as is indicated in Note 54, 

 the shore species sometimes display a tendency in the same 

 •direction. It is described by Hillebrand as occurring "on all 

 islands, in forests up to 2,000 feet." Like those of the inland 

 species of Erythrina (E. monosperma), its seeds sink in sea-water 

 even after being kept for four years, nor could the pods be utilised 

 VOL. Tl ^ 



