Postelstia 73 
some the transition is not as gradual as between 
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 
drenimecaj state Of micration.  ‘ehe “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- 
