OV THE ENOiLISH LAKE lilSTKlCT. 41o 
AVith the physical conditions of the district and its lakes it is not my 
business to deal. The foundation of our knowledge of these lakes was laid 
in the survey of Dr. H. K. Mill (1895), and Dr. Pearsall has, in his papers 
on the aquatic flora and pliytoplankton, added greatly to our knowledge of 
tjie post-Glacial history and present conditions of these lakes. The accom- 
panying map clearly shows relations of the lakes, which occupy valleys 
radiating from a common centre. 
The Plankton. — Comparison oj the various Lakes. 
One of the chief results of Dr. Pearsall's work is that " the stage of 
evolution of the lake-basin must be regarded as being the fundamental factor 
affecting the distribution of the phytoplankton, since it is upon this factor 
that the character of the waters depends " (1921, p. 279), and he has divided 
the lakes into groups on the basis of the amount of silting of their beds. 
These groups are : — 
I. Primitive Lakes. 
Wastwater, Ennerdale, Buttermere, and (h-ummock. 
II. Intermediate Lakes. 
Hawes Water, Derwentwater, Bassenthwaite. 
III. Evolved Lakes. 
Coniston, Windermere, Ullswater. 
IV. Most evolved Lakes. 
Esthvvaite (and (Jrasmere). 
The phytoplankton corresponds fairly well with this grouping, the lakes 
of the first group containing a Desmid plankton, while in the remainder 
there is a large proportion of Diatoms. But Desmids are also frequent in 
Group II., while Myxophycese form an increasing component of the plankton 
of Groups III. arid IV., and are at times dominant in Esthwaite. As 
Dr. Pearsall intends to deal in detail with the plankton as a whole in relation 
to its environment, I shall do no more than attempt to show how far the 
distribution of Entoniostraca conforms to this grouping. 
Any grouping of these lakes on the basis of their Crustacea is by no means 
easy, since, with the exception of Buttermere and Crumraock, which lie in 
the same valley and undoubtedly at one time formed a single lake, each lake 
has its own individual character — so much so that it would probably always 
be possible to identify the lake from which any given representative sample 
of plankton was taken. 
In Table 1, I have arranged the lakes in the groups adopted by Dr. 
Pearsall, Group I. being the most primitive and least silted, and from this 
table the composition of the Crustacean plankton can be seen at a glance. 
Every plankton species taken is recorded in the table, but those which 
occurred only singly in one collection or were evidently not regular con- 
stituents of the plankton are distinguished by a cross enclosed in a circle. 
