178 Proceedings of the Royal Society of Edinburgh. [Sess. 
The laws which govern the geographical distribution of aquatic plants 
cannot be fully understood until science has revealed more facts regarding 
the ecology of the plants than it at present possesses ; it is therefore futile 
to attempt the deduction of general laws, with only an inadequate know- 
ledge of the phenomena to be generalised. During the last great glacial 
epoch it is certain that all forms of the higher plants were banished from 
the greater portion of Scotland. Towards the end of that era, as the mantle 
of ice and snow began to retreat, so would plants encroach again over the 
country from the region to the south, where its influence had been less 
severe. What precise causes influenced most this gradual northward march 
of aquatic and terrestrial plants cannot now be determined, but probably 
they were such as affect the distribution of plants at the present day. 
The plants no doubt followed the lines of least resistance and greatest 
traction, not only in their geographical advance, but also in their adapta- 
tions of structure and function to the varying environments. These lines 
must necessarily be ramified and involved, perhaps to an insoluble degree ; 
yet on them are the secrets of plant geography to be discovered, on the 
basis of physiological anatomy and plant psychology. By such methods a 
most interesting inquiry would be — What is the equilibrium that has heen 
attained between the forces of resistance and traction that has caused 
certain species to arrive at, and remain in, restricted areas ? This is a subject 
bristling with difficult chemical and physical complications, combined with 
the various influences resulting from the never-ceasing action and reaction, 
not only between the different members and associations of the flora, but 
between the flora and fauna as well ; and it is at present impossible to set 
down a complete satisfactory statement of the ecology of any single group 
of aquatic plants. We must, therefore, for the moment leave the final 
generalisation of the causes which govern the distribution of plants, and 
content ourselves with the routine' work of taking evidence of that which 
occurs ; not being too eager to surround ourselves with metaphysical hypo- 
theses which seek to explain the observed phenomenon by a noumenon, nor 
to cloak our ignorance and befool our senses with vague concepts that tran- 
scend actual demonstration, and, when analysed, explain to our intelligence 
nothing whatever. On p. 172 I gave a possible explanation of the introduc- 
tion of Anacharis Alsinastrum into Loch Leven, and of its restriction to that 
loch ; but I am quite ignorant of the exact reason why it should flourish so 
exceedingly there, and not in Loch Fitty, where it also occurs, but very 
sparingly. Again, the restriction of certain plants to particular localities 
may be accounted for by observing that they are ill adapted for any mode 
of dispersal to which they are likely to be subjected ; it is then difficult to 
