1909-10.] Flora of Scottish Lakes. 179 



understand their introduction to their present situation. It is not easily 

 explained why Equisetum limosum, Carex rostrata, Phragmites communis, 

 and others should be so widely distributed about the margins of all kinds 

 of lochs, whereas Cladiuni Mariscus, an equally dominant species, should be 

 restricted, in the Areas under discussion, to a few places in Wigtownshire. 

 When the sub-science of plant-ecology has taught us the full facts regard- 

 ing the relationship existing between organism and enviroment, then shall 

 we be able to generalise sets of phenomena regarding the geographical 

 distribution of water plants to some useful purpose. 



Whilst I have no desire to enter the contest with those who so boldly 

 wield the cudgels in the arena of the origin of species, yet I may brieflj^ 

 state the impression regarding this subject which the study of the plants of 

 the lochs has left upon me. In the first place, it seems to me that aquatic 

 plants have not always had their origin from terrestrial forms that had 

 been forced into the water by more robust competitors on the land, as is 

 sometimes stated, but, more probably, because certain mutable forms have 

 exhibited a tendenc3^ as some do even now, to take on the aquatic habit, 

 that mode of living being more agreeable to their requirements. Some 

 plants form themselves into dense associations consisting of one species only, 

 which spread over considerable areas, and not only prevent others from 

 growing amongst them, but year b}^ year extend their borders at the 

 expense of neighbouring plants. In the vanguard of such colonies there is 

 doubtless very keen competition for the space, and the weaker or less 

 suitably adapted species will be slowly driven before the stronger. This, 

 however, is unlikelj^ to go on continuousl_y, because the stronger species will 

 sooner or later meet with physical or chemical barriers which it is ill 

 adapted to overcome, but to which the weaker species may be better 

 adapted. Quite commonly, it is not that competition for available space 

 is so great, but that tlie local conditions favour the dominant growth of 

 a few individual species. One frequently finds normal terrestrial or 

 marsh species taking on the aquatic habit : instance Ranunculus Flanunula, 

 Juncus supinus, J. acutiflorus, Peplis Portula, etc., but always of their 

 own free will, so to speak, i.e. by the exercise of the subtle power of 

 adaptability which is more or less the common possession of all plants; 

 never have I observed the case of a plant being driven into the water by 

 a stronger competitor. 



From another a.spect of this interesting subject, it appears to me that 

 other causes for variation, with the consequent production of new forms, 

 lie in the fact that although the conditions for plant life are so often 

 remote from the ideal, yet the plastic power possessed by plants, enabling 



