CHAPTER XIII 



ADAPTATION AND SEED-BUOYANCY 



The question of the operation of Natural Selection.— Are there two principles 

 at work ?— The presence of buoyant tissue in the seed-tests and fruit-coats 

 of inland plants, both wild and cultivated. — Useless buoyancy.— The 

 buoyancy of seeds and fruits is not concerned with adaptation. — Summary. 



When we speak of a certain structure as an adaptation to dispersal 

 by currents through the agency of Natural Selection, it is necessary 

 at the outset to be quite clear as to what is implied. Professor 

 Schimper, who brought his great and varied knowledge of many 

 other phases of plant-life to bear on this subject, is careful to clear 

 the ground of preliminary erroneous conceptions in such a per- 

 spicuous and impartial manner that we cannot do better than follow 

 his guidance. There are, he observes (p. 178), many mechanisms 

 or contrivances in plants, which, though they seem to have arisen 

 with a fixed purpose, can in no wise be regarded as having been 

 developed for that end, since they were produced in quite a different 

 connection and have merely acquired a new or supplementary 

 function, of which they are the cause and not the effect. 



This is very much the position that I have taken up for the 

 whole subject of the relation between plants and their dispersing 

 agencies, and it will be found discussed in Chapter XI. It involves, 

 as I venture to think, a dominant principle in the organic world, 

 which it is one of the objects of this work to emphasise, namelyj 

 that Nature in dispersing plants habitually makes use of structures 

 and capacities that were originally developed in quite another 

 connection. Behind this change of function, this new purpose, lies 

 the secret of the organic world. There is for me no more pregnant 

 fact in plant-life than the thistle-seed blown before the wind, or the 

 seed of our sea-convolvulus floating in the sea. It proves to my 

 mind that the evolutionary power in nature is checked and 



