306 THE FRESH-WATEK LOCHS OF SCOTLAND 
ing to the cool depths of the southern lake ; but such an explanation 
is discounted by the animal occurring in Scotland, and especially in 
the northern islands, where the climate is so mild. 
Other Swiss Lakes. — Later investigations by Penard, Zschokke, 
and others have shown that some at least of the species supposed to 
be exclusively abyssal are present in other Swiss lakes. Asellus 
foreli^ Niphargus fo7'eli^ Macroi^hynchus lemajii, Plagiostoma lemmii, 
and many of the Rhizopods, are found in some or in many other lakes. 
Zschokke, in a quite recent work ^ which I have been unable to 
consult, enumerates 100 species which he found in the Lake of Lucerne 
at depths of 170 metres and more. He divides the species into two 
classes : (a) littoral forms, which have migrated downwards ; and (b) 
genuine abyssal forms, which represent relicts of a stenothermal post- 
glacial fauna. 
Forel contends that true abyssal species are completely isolated 
in each lake, and cannot be disseminated from one lake to another. 
Cross-breeding between species in analogous situations in different 
lakes will thus be completely precluded. This view appears to be the 
only one tenable, in the present state of knowledge. Abyssal species 
might have their distribution provided for by means of floating eggs, 
etc. ; but there has been no suggestion that such means of dissemina- 
tion actually exist. Yet there are peculiar abyssal species identical 
in different lakes, completely isolated from one another. 
Forel argues consistently that in each lake the abyssal forms have 
originated independently, in sitit^ and he traces each to the parent form 
among littoral species. If the independent origin in each lake is not 
admitted, an alternative view is that the abyssal forms are survivals 
of a time when conditions were different, when all the lakes where 
they are found were united, or when the climate was such that theSe 
species would find their suitable environment in the littoral region. 
In either case migration would be possible. The abyssal fauna would 
in this view be, like the marine relict fauna, a relict fauna, but surviv- 
ing from earlier fresh-water conditions. 
The study of the Scottish lakes, where the amount of abyssal 
peculiarity is very small, favours Forel's view that the abyssal fauna 
originated in each case in situ. He shows that the amount of modifi- 
cation of the abyssal species is very small, and that they should rank, 
from the morphological point of view, as simple varieties. The only 
species apparently greatly modified in adaptation to abyssal con- 
ditions, the blind Crustacea, Asellus and Niphargus^ are derived from 
the subterranean blind species, and but slightly modified. 
The abyssal species in different lakes are, then, only parallel modifi- 
1 Arch. Hydrohiol. Plcmctonhunde, ii., 1906, Heft 1, pp. 1-8 (quoted in Journ. 
Roy. Micr. Soc, 1907, p. 296). 
