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light by a systematic survey of lakes ? Well, apart from the 

 desirability of having the contours of the lake-basins determined for 

 the purpose of completing our maps and thus showing the exact con- 

 figuration of the earth's surface at the present day, there is the geological 

 interest attaching to the much-disputed origin of lake-basins ; also 

 the increasing interest in the (juestion of water-supply from lakes for 

 our great cities, and the important (juestion of the development of 

 our inland fisheries. These and many lesser problems can only 

 be properly attacked when we have at our command a much more 

 extensive knowledge of a large number of individual lakes than we 

 at present possess. From the strictly biological point of view, more- 

 over, there is one great fact which seems to justify any amount of 

 work and trouble, and that is that a lake is probably the most com- 

 plete and self-contained " unit of environment," as it has been aptly 

 called by Eigenmann, that occurs anywhere in nature. It is, of 

 course, true that some so-called lakes are not at all well-defined 

 pieces of water, but in the vast majority of cases lakes are almost 

 isolated " units," with well-marked boundaries, and offering com- 

 paratively simple conditions of existence. It follows from this that 

 each lake is a microcosm in itself, and, as it is usually of limited 

 extent, it may be studied as a whole with some hope of success. 

 Owing, however, to the infinite variety in the position, age, geological 

 structure, shape, etc., of the lake-basins, and also in the amount and 

 quality of the water and other factors, it also follows that each lake 

 has a distinct individuality of its own. Lakes, in fact, occupy the 

 same position with regard to the waters of the globe that islands do 

 with regard to the land, and their careful study promises even better 

 results than have been obtained in the case of islands, which is 

 saying a good deal. Altogether it seems probable that it will be in 

 a lake, if anywhere, that it will be possible to successfully investigate, 

 under natural conditions, many of the larger biological puzzles, such 

 as the exact influence of the environment, the inheritance or other- 

 wise of acquired characters, the laws of variation, etc. 



Before going on to consider the methods of limnological research 

 a few words may be devoted to a consideration of the question as to 

 what constitutes a lake. In ordinary speech the term "lake" is 

 used very loosely, but there is always an underlying idea that a lake 

 should be a piece of water of considerable area, although it is 

 obviously very difficult to draw a hard and fast line between lakes 

 on the one hand and large ponds, meres, broads, and tarns on the 

 other. Depth is not, as a rule, taken into account, but, from the 

 scientific point of view, this is even more important than area, and 

 students of the subject now usually consider that for a piece 

 of water to be properly called a lake it should not only have an area 

 of, at least, some considerable fraction of a scjuare mile (say, a 

 rjuarter or fifth), but that it should have a depth, over a good part of 

 this area, exceeding the lowest limit of growth of the Characete and 

 aquatic mosses. This depth necessarily varies somewhat in different 



