70 A NATURALIST IN THE PACIFIC chap. 



practically the same as those distributed around the tropical zone 

 which are enumerated in the list given under Note 35, <5. With 

 their home in America, by crossing the Pacific they would ulti- 

 mately arrive at the East African coast, where their course 

 westward would terminate ; whilst commencing their journey from 

 the east side of the American continent they would reach the West 

 African coast ; and their distribution around the tropics of the 

 world would be explained. There follow from these considerations 

 the corollaries that a tropicaFstrand-plant dispersed by the currents 

 which has its birthplace in Asia could never reach the American 

 region, and that American strand-plants are for the most part 

 native-born, excepting those, if there are any, that hail originally 

 from the African West Coast. 



It is necessary in passing to explain the similarity of shore 

 plants on the Pacific and Atlantic coasts of Tropical America. 

 For the mangroves and their accompanying plants inter-com- 

 munication between the two coasts is now impossible ; and a 

 communication between the two oceans must be postulated 

 within the lives of the existing species. For the plants like 

 Entada scandens and Ipomea pes caprae, which occur inland as 

 well as at the coast, it is easy to show that in the case of the 

 Panama Isthmus, their seeds could be readily carried into the 

 Atlantic and Pacific Oceans by rivers draining the opposite slopes 

 of the same " divide," so that the dispersal of the same species from 

 a common centre into two oceans may be seen in operation in our 

 own day. My observations on this subject are given in Chapter 

 XXXII., to which the reader is referred. 



I have now gone far enough to indicate the place that America 

 holds with regard to the distribution of tropical shore-plants dis- 

 persed by the currents and with regard to the currents. There is 

 every probability, as I venture to think I have shown, that the 

 Pacific islands have derived most of their ubiquitous shore-plants 

 with buoyant seeds or fruits from America. But one of the results 

 of our discussion of America in this double aspect was that 

 excepting in the case of the African West Coast it gives but does 

 not receive plants from the Old World. We apply this test, with 

 perhaps a little hesitation, to the shore-plants of the Pacific islands 

 that are dispersed by the currents ; and we find, as will be seen- 

 below, that it is responded to in a remarkable manner. 



It has been observed in the previous chapter that scarcely any 

 of the large-fruited beach-plants of the South Pacific islands, that 

 could only have been dispersed by the currents, have reached 



