1902] SURVEY OF THE HURON RIVER VALLEY 139 



I 3. The strength of the tension line between zones is increased 



\ by the soil-forming activities, because the soil which each zone 



produces makes the lake more unfit for it and fit for the succeed- 



mg zones. 





4. An interaction of organic and inorganic agencies has 

 caused and is now causing an unmistakable advance of plants 

 into the lake, which is gradually being filled by the soil they 

 produce. The more important among these agencies are soil, 

 light, and morphological advantages. 



5. The struggle for existence in each zone is less successful 

 on the landward than on the lakeward side of that zone. 



6. The plants engaged in this sev^ere struggle show a marked 

 tendency to mass themselves in solid ranks. 



Botanical Laboratory, 

 University of Michigan. 



