On the nutritive conditions determining the growth of certain fresh-water and soil protista

It is well known that in ponds and lakes cycles of development occur, in which various kinds of animals and plants replace one another in succession, but the conditions are usually so complex that the succession rarely repeats itself with regularity from year to year, and it is impossible to assign, with any certainty, the successive phases to their determining causes. The same kind of cyclical development occurs in artificially made organic influsion, where bacteria algæ, flagellates and ciliates replace one another in irregular sequence. The object of this paper is to indicate certain lines of experiment upon which it may be possible to attack this problem.