IP 5 t e I s i a 



73 



some the transition is not as gradual as l^etween 

 others, and this fact is observable upon the 

 shores of Lake San Juan. Towards the front 

 the transitions are gradual. In the backstrand 

 there is a sharp passing over from the fringe of 

 willows to the wall of tall, coniferous trees of the 

 forest proper. 



The whole beach formation is however a 

 plastic one. The different societies and groups 

 are in a state of migration. The successive 

 zones are continually, though slowly, moving 

 lakeward as the filling in process goes on, each 

 in turn by degrees coming to occupy the place 

 once tenanted by other forms, all leaving their 

 decaying generations behind, changing, building 

 up and preparing the soil for the permanent 

 occupation by the forest which is here the last 

 link in the chain. 



4. Lake formation. 



The hydrophytic types which are here de- 

 veloped show much the same disposition of their 

 various societies which is characteristic of more 

 northern fresh water lakes. Not taking algal 

 vegetation into consideration and beginning far- 



