10 BULLETIN OF THE NATURAL HISTORY SOCIETY. 



tliat in general it is determined by the fundamental 

 principle of least resistance — the most available or the 

 most easily modified structure is led by the adaptive 

 forces along the easiest lines, and the result is the structure 

 as we have it. 



It is of course necessary in the study of locomotion 

 as elsewhere in phytobiology, to distinguish that which 

 has been adapted from that which is incidental. All 

 gradations between these occur. Without doubt in very 

 many cases the adapted has originated in the incidental- 

 Thus the presence of a wing on a seed is a case of gradual 

 adaptation to wind-locomotion, and the principle of 

 natural selection explains very well how adaptatii)ii may 

 have been perfected. On the other hand the wide 

 locomotion which willows secure by the floating of their 

 brittle and Avind-broken twigs is incidental, though it is 



• easy to understand that if it were worth while this 

 mode of locomotion could be improved and perfected, 

 Examples of structure just on the boundaries of the 

 adapted and incidental are found in some of the modes 



• of vegetative locomotion presently to be mentioned. 



As in the case of many other natural phenomena, 

 the facts of locomotion have long been known, but it is 

 only in this century that the philosophical significance 

 of the fiacts has been understood. That seeds are 

 scattered by wind and animals is plain to observation ; 

 but to know that plants derive advantage from this, and 

 that the advantage in the keen struggle for life to which 

 they are exposed may explain the perfecting and even 

 the origin of the adaptations to secure the locomotion, 

 this is knowledge which has been gained slowly and has 

 l^een made possible only by the light thrown upon all 

 ■organic processes by the principle ot evolution. 



