308 THE FRESH-WATER LOCHS OF SCOTLAND 
abyssal examples do not as a rule differ perceptibly from littoral 
examples. 
What bearing have these facts upon the age of Loch Ness 
relatively to the Lake of Geneva ? The poverty in abyssal species 
and even varieties does suggest that there has been a much longer 
time for the modification of species in the Lake of Geneva than in 
Loch Ness. Slight though the modification in the Lake of Geneva 
may be, it is still less, or practically nil, in I^och Ness. 
There is one factor in the development of species which must be 
considered in comparing these lakes. In the development of varieties, 
and eventually of species, isolation plays an important part. When 
a species migrates into a new environment, selection will at once 
begin to adapt the species to the environment ; but if there is no 
degree of isolation, cross-breeding will prevent or retard the change. 
One would expect the rapiditv of change to depend to a large extent 
on the dcOTee of isolation. In a ffreat lake like the Lake of Geneva 
the first migrants to the central plain might remain long without 
recruits from the littoral. Cross-breeding with more recent migrants 
might be so infrequent as not to retard the action of selection. 
There might thus originate in the central parts of the lake races 
better adapted to the abvssal conditions, which might then gradually 
occupy all the region for which they were specially fitted. 
Whether the actual history of the abyssal fauna of the Lake of 
Geneva in any way corresponds with that theory, in Scotland such an 
origin would be unlikely. The central plain of the lakes is never 
extensive. The steep sides bringing numbei's of involuntary migrants, 
these could readily traverse any part of the central plain, mingling 
with the earlier natives. Migration from the shore might thus give 
rise to no peculiar forms. The facts at any rate accord with this 
theory. 
Fauna Relicta. — None of the marine relicts, or more recent 
migrants, have been found in the Scottish lochs. We had on Loch 
Ness, Loch Lochy, etc., the benefit of the assistance of Dr Wesenberg- 
Lund, who was acquainted with the relict fauna in the Danish lakes, 
and who used the same apparatus and methods in Scotland which 
were successful in obtaining the relicts in Denmark. Mysis has been 
found in several lochs, but those were all very close to the sea and 
near sea-level, so that migration would be easy. Those examples 
which have been determined were all M. vulgaris, never M. relicta. 
Origin of the Scottish Lacustrine Fauna and Flora. — The deriva- 
tion of the whole lacustrine population presents no difficulty, since it 
has been possible to trace the abyssal fauna directly to the littoral, 
without even perceptible modifications. The plankton is a mingling 
of the common European species with Arctic species. The cosmo- 
