XIII ADAPTATION AND SEED-BUOYANCY - 123 



two plants belonging to the adaptive group, Professor Schimper 

 (pp. 156, 188) admits also the dispersing agency of frugivorous 

 birds, and he claims it for Morinda citrifolia, m the pyrenes of 

 which he also detects a special adaptation to dispersal by currents. 

 It may be added that, as he also points out, fruits of the non- 

 adaptive group of littoral plants, such as Premna integrifolia 

 (P. taitensis) and Cassytha filiformis, would sometimes also attract 

 birds. In fact, those of the last-named have been found in the 

 crops of pigeons {^Introd. Chall. Bot., p. 46). 



Looking at all these littoral plants with fruits that are equally 

 fitted for dispersal by birds and by currents, we may now ask, 

 Where does the general principle of adaptation to dispersal lie ? 

 Whatever view w^e adopt, we must apply the same view to all, 

 whether it be a question of dispersal by birds or by currents. We 

 cannot choose between two sets of principles determining the 

 buoyancy of seeds and fruits any more than we can regard a 

 fleshy drupe and a buoyant seed as illustrating different principles 

 regulating the dispersal of plants. Nature works with uniformity 

 in these matters, and if the Natural Selection theory is held to 

 explain one case it ought to account for all. Yet nobody would 

 go so far as this ; and this view of dispersal is on many grounds 

 antecedently improbable. These difficulties disappear if we assume 

 that in all cases the dispersing agencies have without modification 

 made use of characters and cajDacities that were developed, as we 

 now see them, in quite other connections and under quite other 

 conditions. 



It will now h^ necessary to look a little closer into the subject 

 of the buoyant tissue, to the existence of which in their coats about 

 half of the littoral plants concerned owe the floating power of 

 their fruits or seeds. In the first place, it is to be remarked that in 

 the case of some of the seeds of the plants of the non-adaptive 

 groups it is also represented to a small degree in the seed-coats, 

 although, as with Strongylodon lucidum and Mucuna urens, it is 

 not present in sufficient amount to float the seed. In the next 

 place, it should be noted that with some genera possessing, like 

 Terminalia, both inland and coast species it is to be found alike in 

 the fruit-coverings of inland and of littoral plants, though in a less 

 degree in the case of the fruits of inland species, the floating power 

 of which is proportionately diminished. There are, however, a 

 few cases where this buoyant tissue is developed in inland species 

 which belong to genera or subgenera that have no littoral species. 

 This is what we would expect, if Natural Selection has merely 



