84 PLANTS, SEEDS, AND CURRENTS 



by currents ; and the same may be said of those genera without littoral 

 species where there is no connection between America and the Old 

 World, as in the case of Mammea with six known species, of which 

 three are restricted to tropical America and three are known only 

 from Madagascar. Here we are concerned with the original dis- 

 tribution of the genera in the tropical zone, and the presence or 

 absence of any link between America and the eastern hemisphere 

 seems to be a matter of fitness for dispersal by currents. One can 

 also understand cases like Terminalia, where the absence of any 

 species linking the two worlds may be concerned with the circum- 

 stance that the littoral species capable of being spread by the currents 

 are not found on the West African coast ; but the behaviour of the 

 genera, Symphonia and Crudya, is not easy to comprehend, since 

 there we have one genus almost entirely of the Old World with only 

 a single outpost in America, and another almost entirely American 

 with a distant representative in the Philippine Islands. 



The important lesson to be learned from the discontinuity of 

 genera in tropical latitudes is that we can only appeal to the currents 

 in the case of the littoral species, the original distribution of the genus 

 around the tropical zone being quite another matter. Facts of this 

 kind go far to limit the sphere of influence of the oceanic current in 

 determining plant distribution. Currents have done little to con- 

 fuse the great issues raised by the genera. Whilst they have often 

 effected the mingling of littoral floras of continents, the main facts 

 of distribution are largely undisturbed. With these introductory 

 remarks we will now proceed to discuss the similarity of the West 

 Indian and West African littoral floras, and in so doing we will take 

 up again the story of the beach-drift. 



An Appeal to the Oceanic Currents. — Perhaps the most im- 

 portant question raised by the study of West Indian beach-drift is 

 suggested by the fact that more than half of the plants that con- 

 tribute to it (beach plants, mangroves and their associates, estuarine 

 plants, and inland plants that grow at times at the riverside) occur 

 outside the New World. Almost all of the Old World plants here 

 concerned have been recorded from West Africa; and there can be 

 little doubt that the few exceptions, most of which have been 

 observed on the east side of the continent, will disappear with future 

 inquiry. 



This fact at once leads one to investigate the relation between the 

 occurrence of these West Indian plants in West Africa and their 

 suitability for dispersal by oceanic currents. To each of these 

 plants, as well as to those that are restricted entirely to the New 

 World, the question has been put, whether or not its fruit or seed, 

 as the case may be, could be transported without loss of the germina- 

 tive power in the Main Equatorial Current from West Africa to Brazil. 

 This, as is shown in Chapter III., is the shortest available route 

 between the two Worlds for the transference by currents of floating 

 fruits and seeds. It requires a capacity on the part of the fruit 

 or seed of floating unharmed from two to three months in sea- 

 water, and of being able to withstand the ordinary buffeting of the 

 waves. 



