304 THE FRESH-WATER LOCHS OF SCOTLAND 
ON THE BEARING OF THE BIOLOGICAL EVIDENCE AS TO 
THE ORIGIN AND AGE OF THE SCOTTISH LOCHS 
We cannot hope to derive from the study of the biology any 
positive information as to the mode of origin and possible age of our 
lakes. We look to geology for approximate answers to such questions. 
When geology has pronounced upon them, we may examine the 
biology in its bearing upon the supposed origin and age, and seek 
confirmation or the reverse. 
If the lakes are excessively ancient (geologically speaking), we may 
reasonably expect a fauna and flora rich in peculiar forms. If they 
are but of yesterday, we may expect a fauna and flora easily derived 
from surrounding regions. If it is admitted that there have been 
several glacial periods, separated by long intervals, it may be that 
some of the Scottish lakes are of respectable antiquity, considered 
merely as lakes or depressions in valleys. As, however, each glacial 
period would interrupt the life of the lake, destroying the individuals 
and annihilating any peculiar species which might have originated in 
it, it follows that, as a biological entity, each lake dates only from 
the termination of the last glacial period affecting the region where it 
is found. Again, if there has been, since the last glacial epoch, suffi- 
cient elevation of the land to convert depressions of the sea-bottom 
into fresh-water lakes, such lakes, in respect of the investigation into 
fresh-water life, date only from the time when fresh water replaced 
salt in the basin. If any lakes have originated in the latter way, 
there might be found survivors of the marine fauna which formerly 
occupied the basin. 
Comparison with other European Lakes 
Various European lakes possess what are supposed to be survivals 
of a marine fauna or a peculiar fresh-water abyssal fauna. 
Lake of Geneva. — As the most fully studied of European lakes, 
we will consider that lake most carefully. Professor Forel enumerates 
79 species. Excluding vertebrata, which we cannot compare with 
his, there are 65 species of abvssal animals. As his abyssal region 
begins very near the surface, and the majoritv of the species are not 
peculiar to it, we are only concerned here with the small number of 
peculiar species. On the most conservative estimate, about 20 of 
those are peculiar species, discovered for the first time in the Lake of 
Geneva, many of them restricted to that lake, and supposed to be 
specially adapted to abyssal conditions. They are chiefly Crustacea, 
Mollusca, and Worms. 
In dealing with these facts there are many reasons for being 
