XIII ADAPTATION AND SEED-BUOYANCY 127 



the application of the Darwinian view to the general subject of the 

 buoyancy of the seeds and fruits of littoral plants lies in the cir- 

 cumstance that quite half of the plants concerned are admitted to 

 be outside the scope of the theory, and that for these another 

 explanation has to be found. I think we may fairly claim that in 

 a matter which finally resolves itself into a question of buoyancy 

 one explanation should cover all. We have thus to decide whether 

 to regard as adaptations to dispersal by currents the structures of 

 the buoyant seeds and fruits of littoral plants ; or whether to hold 

 the view that as far as dispersal by currents is concerned such 

 structures are purely accidental, and that Nature has never directly 

 concerned herself in the matter at all. The first explanation lies 

 under the disadvantage above alluded to, and it remains to be 

 learned whether the second view could be made to cover all cases 

 of dispersal by currents. Further investigation on many points 

 is yet required ; but, apart from the evidence against Natural 

 Selection as the principal agency that has been produced in this 

 chapter, a powerful argument in favour of the view that the 

 buoyancy of seeds and fruits is not concerned with ■ adaptation 

 is, that as a rule the floating capacity of the seed or fruit has 

 no direct relation with the density of sea-water. Generally 

 speaking, as shown in Chapter X., these seeds and fruits are 

 much more buoyant than they need to be, that is to say, if 

 they owe their floating power to adaptation to dispersal by 

 currents. This is quite in accordance with the argument deve- 

 loped in Chapter XI. with regard to the general question of 

 plant-distribution, that dispersing agencies make use of characters 

 and capacities of seeds and fruits that were never intended for 

 them. 



Summary of the Chapter. 



(i) There are many mechanisms or contrivances in plants 

 that now serve a purpose for which they were not originally 

 developed. 



(2) Of this nature, it is contended, is the relation between fruits 

 and seeds and the agencies of dispersal. 



(3) If, however, the structure or mechanism is made more 

 effective by the new function, such a modification may be regarded 

 as an " adaptation " in the language of the theory of Natural 

 Selection. 



