i6 A NATURALIST IN THE PACIFIC chap. 



wide on the tropical coasts of the Old World, and sometimes also 

 in the New World. In more than half the species we are con- 

 cerned with the dispersal by currents of more or less dry inde- 

 hiscent fruits that range usually in size from a marble to a 

 cricket-ball, as illustrated by those of Hernandia peltata and 

 Barringtonia speciosa, whilst with most of the rest the currents 

 distribute large seeds, several of which are Leguminous, as in the 

 case of Mucuna, Caesalpinia, and Entada, with others of the Con- 

 volvulus type, as in the instance of Ipomea pes caprse. It is 

 remarkable that in selecting plants with buoyant seeds or seed- 

 vessels for a station at the coast Nature has generally ignored 

 those with very small seeds. When such small seeded plants, as 

 Sesuvium portulacastrum, occur on the beach, the seeds have as a 

 rule no buoyancy. Pemphis acidula is, however, an exception ; 

 but its case is a very rare one. It will be established in the next 

 chapter that the non-buoyancy of small seeds is generally true also 

 of plants growing by the river or by the pond. 



The point at which we have arrived in our inquiry concerning the 

 general collection of seeds and seed-vessels that we placed in sea- 

 water is that the plants with buoyant seeds or seed-vessels have been for 

 the most part "located" at the coast. But if we look a little more 

 closely at the sunken and floating seeds, we find that in the same 

 genus there are species with seeds or seed-vessels that sink and 

 species with those that float. We look again and then perceive that 

 the same general principle is true of different species of the same 

 genus growing inland and at the coast. We learn now that as a rule 

 when a genus possesses both littoral and inland species, the seeds or 

 fruits of the former float in sea-water for a long time, whilst those 

 of the latter have little or no floating power. But we have yet to 

 examine the structure of the coverings of the buoyant seed or 

 fruit ; and we shall then discover that the different behaviour in 

 water is often associated with corresponding structural differences 

 of a striking character. The structural causes of buoyancy are 

 dealt with in Chapter XII.; and we will now content ourselves 

 with enunciating the second principle that in a genus comprising 

 both coast and inland species, only the coast species possess buoyant 

 seeds or seed-vessels. 



The important principle above indicated was not altogether new 

 to me, as is shown in the next chapter. But it was new in the case 

 of the floras of the Pacific Islands. When it first presented itself in 

 Hawaii I was engaged in trying to find a connection between the 

 inland and littoral species of Scsevola ; and its discovery led me 



