6 2 A NATURALIST IN THE PACIFIC chap. 



It is essential to bear in mind at the outset that for their inland 

 plants the Pacific islands can draw on the floras of a relatively- 

 large portion of the globe. Such plants, having as a rule fruits or 

 seeds that sink in sea-water, or are incapable of floating for long 

 periods, could only have arrived at these islands, where man's 

 interference is excluded, through the agencies of winds and birds, 

 assisted by other lesser agencies, as those of bats, insects, &c. On 

 the other hand, for their littoral plants, which are for the most part 

 dispersed by the currents, the source of supply is very restricted. 

 The shore-plants with buoyant seeds or fruits of the islands of the 

 tropical Pacific, that are here dealt with, number only about seventy, 

 and it is not likely that this number will be greatly increased, since, 

 whatever may be the deficiencies in our acquaintance with the inland 

 floras of these islands, we have a fairly complete knowledge of the 

 strictly littoral plants. 



I do not suppose, indeed, that the number of such plants with 

 seeds or fruits capable of being transported unharmed over wide 

 tracts of sea would much exceed ioo for the whole Indo-Pacific 

 region from India to Tahiti. Professor Schimper gives a list con- 

 taining 117 tropical plants distributed far and wide over the shores 

 of this region, and made up of species dispersed by currents, birds, 

 and man. Taking a liberal estimate, not over two-thirds of the 

 plants mentioned in this list are dispersed by currents. Then, 

 again, if the flora of a coral atoll, like that of Diego Garcia or of 

 the Keeling Islands, is taken as affording an index of the work 

 of the currents, the number of plants dispersed by the currents 

 would appear to be indeed restricted, since in either case their in- 

 digenous flowering plants, including those of both the buoyant and 

 non-buoyant groups, do not exceed fifty. 



About twenty years ago, Mr. Hemsley, who, in his work on 

 the botany of the Challenger Expedition, prepared the way for the 

 investigation of this subject, made a list of not less than 120 plants, 

 almost all tropical, that are " certainly or probably dispersed " by 

 the currents (Introd. Chall. Bot., p. 42). This is admittedly only 

 a preliminary list, and as the result of recent investigations some 

 plants have to be omitted and others to be added ; but I doubt 

 whether, numerically, it is far below the mark. The relative 

 efficacy of the currents seems to have been first systematically 

 discussed by De Candolle in his Gdographie Botanique, which 

 was published in 1855. Data were then very scanty, and out of a 

 list of nearly 100 inter-tropical species (Old World plants found in 

 the New World and New World plants found in the Old World) 



