544 THE AMERICAN NATURALIST [Vol. XLVI 



What, then, is the relationship— the physiological and 

 genetic relationship — between these two types? Jen- 

 nings in his recent work on the "Characteristics of the 

 Diverse Kaces of Paramecium" has prophesied that the 

 more exact study of the life history of rotifers will 

 demonstrate that much of their apparent variability is 

 really due to the presence, within specific limits, of 

 numerous fixed races. 



Now the study, as here outlined, of the variation of A. 

 amphora, brings to light a condition- which in no wise 

 substantiates this prophecy. No fixed races are present ; 

 but strongly demarcated yet temporary types, on the one 

 hand, and fluctuating variations, on the other, which are 

 all or nearly all the result of nutritive stimuli. Is it 

 possible that, in spite of this, the closely related A. 

 bright w<-lU will present the fixed races which Jennings 

 suggests? 



I have aleady indicated that a series of about twenty 

 culture experiments with the type of A. bright ircUi first 

 found by me yielded no significant modification. At the 

 present writing I am again following this species in 

 copious natural development and again conducting a 

 few mass cultures without finding anything but farther 

 proofs of constancy. 



I am also succeeding in rearing very large numbers 

 of A. bright it elli of the type whose trophi present the 

 inner tooth. The resting eggs, which were brought from 

 Sylvan Lake the preceding August, were kept over winter 

 in a small amount of the lake water and hatched out in 

 March by adding tap water and raising the temperature 

 by placing the dish in the sunlight. The culture medium 

 has slowly been quite changed to the somewhat alkaline 

 and saline water of the writer's locality. The cultures 

 have also been heavily fed upon organisms to which they 

 are certainly unaccustomed in nature. Some cannibal- 

 ism has been induced. But the trophi remain obstinately 

 true to their own type, and the general morphological 

 changes have been confined to a considerable increase in 



