160 THE FRESH- WATER LOCHS OF SCOTLAND 
development, such plants frequently flower less profusely than would 
be the case were they living in still water, owing to the antagonism 
existing in all organic beings between the vegetative and the re- 
productive systems. 
From certain points of view plants may increase in interest and 
value in ratio to their rarity ; of equal worth philosophically are 
those plants that occur in great abundance. The former being 
scattered as individuals, or as small associations over restricted areas, 
are possibly at present of but small import in the economy of nature. 
The commoner plants, however, by reason of their dominance and 
abundance, become important agents, not only as a plant-covering 
to the earth, but also in the effect they produce in the physiography 
of a country : barren tracts become heath or forest by the extension 
of vegetation ; lakes are converted into morasses, moors, or even into 
land suitable for agriculture by the accumulation of plant-remains. 
Such natural operations tend to increase the wealth and social 
prosperity of man. As examples on the other hand, the sudden 
increase of a baneful fungus may bring ruin to thousands of 
agriculturists, and carry famine to the million ; or morasses in hilly 
districts may slide into cultivated valleys, and completely overwhelm 
sites of human activity and wealth. These and many other phenomena 
are brought about by the predominance of certain classes of plants. 
How great, therefore, are the interests awakened upon the fields of 
practical thought and knowledge by the abundant and dominant 
plants in their never-ceasing antagonism with one another and with 
other forces of nature ! 
The investigations described in the pages that follow were under- 
taken chiefly to show which are the dominant plants, of higher 
organisation, in some of the lochs of Scotland, and their distribution 
therein ; together with a few observations upon the leading factors 
that control the growth and extension of such plants. 
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 un- 
doubtedly they were such as affect the distribution of plants at the 
present day. The plants, no doubt, followed the lines of least resist- 
ance and greatest traction, not only in their geographical advance but 
also in their adaptations 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- 
