THE AMERICAN NATURALIST [Vol. XLVI 



From the surface of a pond several feet in depth, and at 

 a point several feet from the shore, a large quantity of 

 the humped type were collected with a small net. This 

 material and the entire pond, so far as my investigations 

 had gone at this time, had shown none of the larger form, 

 and the care with which the material was taken from the 

 surface rendered it improbable that a resting egg of the 

 larger form should contaminate it, the resting eggs of all 

 these rotifers being heavy and sinking at once. 6 This 

 material was placed in clean transparent jars of several 

 liters, although beside each larger culture was usually 

 placed a smaller one in a tall thin vessel which permitted 

 careful scrutiny of the entire contents with a low-power 

 lens. It was almost impossible to crowd the animals to 

 the point of injury, though perhaps every other visible 

 organism would die. Now in all of these crowded mass- 

 cultures, whether large or small, whether fed, half-fed, 

 or starved, the campanulate form made its appearance 

 within at most a week. Sometimes a single individual, 

 either young or fully developed, would first be discov- 

 ered; more frequently a number would be present before 

 noticed. But in every instance the sudden appearance of 

 the large form was followed by its very rapid multiplica- 

 tion, coincident with a still more rapid diminution in 

 numbers of the humped form. The latter were eaten up 

 by the former, even adults of the humped type falling 

 victims to the prodigious ingesting power of the campan- 

 ulates. This all but complete displacement, in the 

 course, say, of three weeks, of one form by the other, 

 was a striking and almost astonishing spectacle. I have 

 not observed it in nature, however crowded the species 

 becomes in its natural situation. But in my culture 

 dishes it was the regular occurrence in case the individ- 

 uals became sufficiently numerous. 



