292 ECOLOGY [CH. xxm 



of the same waters. These changes seem to be much more 

 notable and rapid than those occurring among terrestrial plants 

 in corresponding periods. In certain dykes and ditches, which 

 the present writer has had under more or less continuous obser- 

 vation for some years, various species appear, disappear, and 

 reappear, in a fashion which seems at first glance wholly erratic, 

 but which might, on thorough study, yield results which would 

 throw some light upon the problems of dispersal and distribu- 

 tion. 



