282 



Mr. W. H. PearsalL 



(i) the silting up of the stony feeding grounds or their covering with algae- 

 (e.g., Cladoiohora), as suggested by Weiss ;* (ii) the development of summer 

 " stagnation " and a deficiency of oxygen in the deeper waters of the evolved 

 lakes. Both of these factors are apparentlj^ in operation, and both are 

 dependent upon the other physical characters of the lake basins. Thus, 

 though the intermediate links in the causal chain await further study, it is 

 probably safe to assume that the changes in type and numbers of the tish in 

 these lakes are intimately correlated with the physical development of the 

 lake basins. 



Summary. — The data presented show that a very close conuection exists 

 between the flora (and fishes) of the English Lakes and the physical and 

 chemical conditions of their shores and waters. These lakes are of the same 

 age (glacial), of similar origin, and lie among rocks possessing relatively 

 uniform characters. It is therefore possible to attribute the differences they 

 show to variations in the rates of erosion and sedimentation of the lake 

 basins, due to inequalities in the durability of the underlying rocks. In 

 distinguishing rocky from relatively silted lakes, a contrast is therefore made 

 between primitive and more highly evolved lakes, and it becomes possible to 

 describe the stages in the post-glacial development of a rock-basin. The 

 conception has particular value biologically, since it permits the study of 

 the post-glacial development of vegetation. The two extreme pliases, here 

 distinguished, differ in the following particulars : — 



Extreme Lake Phases. 





Primitive. 



Evolved. 



Percentage of drainage system cultiyable 



„ Fotamogetori and Naias 



Pish 



5-8 

 73-47 

 85-74 



8-5 



*-2 



Desmids dominant 

 Chiefly trout 



17-45 



27-12 



52-30 

 2-0 . 



30-55 

 Diatoms dominant 

 Chiefly perch and 



pike 





Evidence is adduced for considering all these differences as ultimately dependent upon the 

 physical condition of the lakes. 



* Weiss, F. E., " Preliminary Account of the Submerged Vegetation of Lake Winder- 

 mere, etc.," ' Proc. Lit. and Phil. Soc, Manchester,' vol. 53 (1909). 



