216 PLANTS, SEEDS, AND CURRENTS 



trees, it would not appear that either man or the currents would of 

 themselves alone determine the great difference in range. I have 

 shown in my previous work that capacity for dispersal by currents 

 is not of itself sufficient to give a plant a wide range. Behind this 

 capacity must lie a littoral station, and behind that again a xerophilous 

 habit. Whilst H. tiliaceus is to be placed with the xerophytes of 

 the sea border and of the dry inland plains, H . elatus associates with 

 the hygrophytes of the wet woods of the interior. Though both 

 may be found at the riverside, the first is confined to estuaries 

 within tidal influence, whilst the last frequents the higher reaches 

 where the river traverses wooded districts and where mixture with 

 salt water would not be expected to occur. 



These two allied species further illustrate the principle, already 

 alluded to, that whilst coast plants are often spread over a wide area 

 of the globe, inland plants of the same genus are restricted to a 

 limited region. Mere capacity for dispersal by currents would not 

 bring this about unless it concurred with a littoral station. The 

 seeds of Hibiscus elatus, after being carried down by a river from 

 the interior of an island to the sea, would find no suitable station 

 when stranded by the currents on some neighbouring coast. It is 

 the station at the coast that enables the seeds of H. tiliaceus to 

 establish the plant when transported to another shore. 



I come now to deal more especially with Hibiscus tiliaceus. This 

 is one of the " problem " plants of distribution, which, like Acacia 

 farnesiana, Thespesia populnea, etc., are found all round the tropics 

 and are as a rule littoral in station. With this tree, as with Thespesia 

 populnea, it is the littoral station that determines the effectiveness 

 of the currents in dispersing the seeds. But other agencies of dispersal 

 cannot be excluded, such as the influence of man, and, for local dis- 

 tribution, the intervention of birds and other animals. Many details 

 of its station, distribution, and dispersal in the Pacific are given 

 in my book on Plant Dispersal. Here I will mainly confine my 

 remarks to its behaviour in the West Indies. 



This was one of the plants in which De Candolle took special 

 interest. In his work on geographical botany (p. 769), whilst recog- 

 nising man's agency in its dispersal, he suspects that currents were 

 originally effective agents in its distribution, and perhaps at a very 

 ancient date. Not knowing whether it was most common in the 

 east or in the west, that is, in the tropics of the Indian Archipelago 

 or in those of America, he does not at first assign it a home in either 

 hemisphere. However, in a later page (p. 792) he places it with 

 Acacia farnesiana amongst plants spread by the currents, but prob- 

 ably American and naturalised in Asia and Africa. I scarcely think 

 that a purely American origin can be sustained. It doubtless attained 

 its present distribution ages ago, and may have witnessed the emerg- 

 ence of primeval man around the tropics of the globe. Under such 

 circumstances speculations as to its home seem futile. 



Though found in coast regions in all tropical latitudes, on both 

 sides of the American and African continents, in Indo-Malaya, in 

 Australia, and Polynesia, it seems to be less frequent in the New 

 World ; but this is a matter that requires further investigation. It is 



