ANIMAL ORGANISMS IN RELATION TO THEIR ENVIRONMENT 481 



life, especially involving the forms most dependent upon vegetation, and 

 thus there follows a succession of animals represented by more or less 

 clearly defined stages and ending in a practical balance. 



The glacial lakes of the northern part of this country have exhibited a 

 definite series of changes which may be likened to the periods in the life 

 of an organism and permit us to speak of any particular one as a young 





S3 



I. 

 -§ c 



y> 



Pines 





■^ 





5 



Alc^ers 





.ts 



II 





^^^ 



<o ^ 



I 



I 



^^t^ 



Flowering Herbs, 

 Sphagnum Moss, 

 Tamarack. £r IV/7/ow 



Marsh 

 Grasses 



I 



i 



^ -fc > 



Floating 

 Bunches 

 of Grass 



I 



to lO 



Open 

 Wafer 



SO 



Original Boffonr' 



Fig. 301. — Diagram to illustrate succession in a bog lake which is advanced in the 

 process of being filled in. The grading in the shading under the open water indicates the 

 loose, flocfulent deposit which at the top offers no resistance to penetration by a pole or 

 other object, but which becomes more and more dense as the bottom is approached. The 

 animals in each zone are represented by only a few types selected to show variety. 



lake, a lake of middle age, or an old lake. At the beginning of this 

 process the lake has clean sand and gravel bottom and shores. Later 

 plants appear and by their growth, death, and decomposition, and by 

 the washing of soil into the lake, there are gradually developed deposits 

 of mud and vegetable mold on the bottom or in quiet places along the 

 shore. Sand and gravel are then left exposed only on beaches where 

 combined wind and ice action prevent the accumulation of either vegeta- 

 tion or mud. In time, however, there is built out from the shore a mass 

 of vegetation which produces a bog or marsh; at the same time the lake 



