SUMMARY OF OUR KNOWLEDGE 
REGARDING VARIOUS LIMNOLOGICAL 
PROBLEMS 
By Dr C. WESENBERG-LUND 
CONTENTS 
PAGE 
Introduction ........ 374 
Part I.— Contribution to the General Geography of the Lakes 375 
Arctic lakes— North European lakes — Baltic fresh- water 
lakes — Central European alpine lakes — Tropical lakes — 
General remarks. 
„ IL— The Plankton Communities, their Geography and Life- 
History . . . . . . .399 
Explanation of the cosmopolitanism — Means by which the 
cosmopolitanism is brought about : {a) different modes of 
reproduction ; (6) variation, seasonal and local — Influence 
of the Ice Age on the fresh- water plankton — Summar3\ 
„ III.— Main Problems of Future Limnological Investigations 426 
„ IV. — Bibliography ....... 433 
INTRODUCTION 
On a visit to Copenhagen in July 1909 Sir John Murray asked me to 
give him my views on matters connected with fresh-water lakes, their 
variations with latitude, and other physical and chemical conditions, 
as also regarding the variation of the organic life, especially the 
plankton, from pole to pole. 
It must be remembered that a detailed review of the variations 
in the outer conditions {i.e. variations in the physical and chemical 
conditions of the lakes) from the pole to the equator would be the 
same as an account of the general geography of the lakes ; this can 
hardly be done satisfactorily at present, at any rate not by the 
author, as he does not have sufficient mastery over all the elementary 
arguments on which an account of our present knowledge is necessarily 
dependent. Further, it must be remembered that our knowledge of 
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