Flora of Scottish Lakes. 
179 
1909 - 10 .] 
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 Cladium 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 briefly 
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 tendency, 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 by 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 unlikely to go on continuously, 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 the 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 Flammula, 
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 aspect 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 
