BIOLOGY OF THE SCOTTISH LOCHS 
309 
politan species may enter the lochs by ordinary migration. It is 
probable that if the plankton could be annihilated it would be 
replaced by ordinary migration within a few years. Eggs and spores 
of many of the species can be dried up without injury, and may be 
carried throua;h the air as dust from one lake to another. Others 
which could not bear desiccation might be conveyed among mud 
adhering to the feet of aquatic birds, and in various other ways. So 
great is the facility with which plankton species can be transferred 
from one lake to another, that the chief difficulty has been to account 
for the restricted distribution of certain species. 
An experiment, which by good fortune we were able to make in 
Scotland, illustrates the rapidity with which plankton may find its 
way into a loch. A large artificial lake having been constructed, 
several miles in length, of considerable depth, and separated by many 
miles from any other lake, an examination of the water was made 
several times during the first year. In the course of a few months 
many of the cosmopolitan species found their way in, and increased 
till they were as numerous as in an old-established loch. 
The Arctic species in the Scottish plankton might be derived from 
Scandinavia by ordinary migration. Considering the great extent of 
sea which now separates the two countries, the probability of direct 
migration is slight, and a more reasonable theory is that they are 
survivors from a period when Arctic conditions prevailed over a great 
part of Europe. 
The littoral fauna and flora may be equally readily disseminated, 
as they are more constantly liable to desiccation than the plankton, 
and more of the species may be protected against annihilation by the 
production of resting eggs or spores. A large proportion of the 
littoral population is not exclusively aquatic, but belongs to that 
extensive family supported by mosses and similar plants, the members 
of which resume their activity every time the moss is moistened. 
Such organisms need not migrate directly from lake to lake, but may 
find intermediate resting-places anywhere. 
Wallace, in his Island Life, makes a review of the biology of 
the British Isles, and remarks on the very small number of endemic 
species (the Red Grouse, one Moss, and so forth). The numerous 
species or varieties of Salmonidae, restricted to certain lochs, are 
referred to, in relation to the question of the length of time that 
Great Britain has been an island. The duration of the insular 
character of Great Britain is quite apart from the duration of its 
lochs. These may be older than the island. Fresh- water lochs 
being isolated by as efficient a barrier as the sea would be, the 
existence or non-existence of the English Channel does not in any 
way affect them. 
