OF THE ENGLISH LAKE DISTRICT. 419 
it in tliis connection. It is found onlv in the summer months and lias an 
extremely short lite-cycle, a fact which has probably largely assisted it to 
accommodate itself to a great variety of waters. 
Leptodora kindti, although in deep lakes it may have a preference for the 
lower or colder strata, is by no means restricted to deep lakes, nor is it 
characteristic of northern regions. It may rather be regarded as a southern 
element in the fauna. 
Although the crustacean jjlankton of the Lake District has a distinct 
northern facies, the absence i'vom it oi. Diajytomitslaciniatus and D. wierzejskii, 
which are not uncommon in Scotland, is noticeable. The latter is a eurytherm 
species of remarkably wide range, but characteristic of pools rather than of 
lakes, but the former is a definitely arctic-alpine species. Diaptomus laticeps 
is a member of the group of northern Diaptomus which is widely distributed 
in Scotland, but only occurs in this district in Hawes Water and Goats 
Water. It seems probable that it may be a recent immigrant. 
From the point of view of the Orustacea the Lake District plankton 
certainly belongs to the Scottish Highland type, with the addition of a few 
southern forms. 
It must be admitted that any discussion as to whether the plankton of any 
lake or district has a northern or southern facies is of doubtful value in the 
present state of our knowledge of the means of distribution and required 
conditions of existence of the different species. Not only is the whole 
question of the relation of the present fauna to the effects of the Glacial 
period far from being clear, but we have little means of knowing to what 
extent the fauna was able to survive that period in its original habitat. In 
the case of the Lake District it seems fairly certain that the jaresent lake-. 
basins were filled with ice, and it is probable that the whole district was 
covered with a mantle of ice, so that the present aquatic faima is entirely of 
post-Glacial origin. It has been re-colonised largely by northern species, 
because these species find here, under a lowland climate, the conditions 
necessary for their existence, which are depth of water, form of lake-basin, 
and chemical composition of the rocks of the drainage area. The similarity 
of the plankton of the Scottish and Cumberland lakes, and of both with 
that of some Scandinavian and Irish lakes, may be due in large part to their 
being excavated in ancient lime-free rocks rather than to climatic influences. 
The similarity between the arctic and alpine fauna of Entomostraca may be 
accounted for by unlimited powers of dispersal rather than by the influence 
of the Glacial period. It is true that the evidence from other groups is in 
favour of the current views as to the relation of the alpine to the arctic 
fauna, but we require to know far more about means of dispersal before the 
matter can be regarded as decided. I will give two instances within my 
own knowledge which illustrate the danger of relying on limited powers of 
dispersal and upon the nature of the habitat in speculations regarding 
