THE AMERICAN NATURALIST [Vol. XLVII 



(1) The rate of reproduction of P. aurelia and P. caudatum is in- 

 fluenced by the volume of the culture medium, within the limits tested 

 [». e., 2, 5, 20 and 40 drops of varied environment medium] and the 

 greater the volume the more rapid is the rate of division. (2) Par- 

 amecia excrete substances which are toxic to themselves when present 

 in their environment, and these substances are more effective when the 

 organisms are confined in limited volumes of the culture fluid. (3) The 

 excretion products of Paramecium play an appreciable part in de- 

 termining the period of maximum numbers, rate of decline, etc., of this 

 an 1 1 f u (12. page 581). 



A careful study of the effects of changes of temperature on 

 reproduction made by Woodruff and Baitsell (17) showed that 

 the temperature coefficient (factor by which the rate of repro- 

 duction is multiplied when the temperature is raised ten degrees) 

 is approximately 2.7, so that the rate of cell division is influenced 

 by the temperature in a manner similar to that for a chemical 



These studies of environmental action had shown Woodruff 

 that different races of Paramecium are adapted to different con- 

 ditions, and that this throws light on the diverse results reported 

 by different observers. In a paper of 1911 (18), he concludes: 



(1) The discrepant results of various workers on the longevity of 

 Paramecium are in all probability due to variation in the cultural 

 demands of the race isolated for study. (2) It is probable that most, 

 if not all, normal individuals have under suitable environmental condi- 

 tions, unlimited power of reproduction without conjugation or artificial 

 stimulation (18, page 65). 



In this second statement much is involved in the word 

 "normal"; the experience of the Johns Hopkins Laboratory is 

 that some lines die out after a time, even though they may at 

 first multiply in the usual way. 



The study of cultural action was next made general and 

 extended to the other organisms in the infusorian cultures, by a 

 careful examination of the source and sequence of development 

 of the organisms usually found in the cultures of decaying vege- 

 tation. The conclusions are of practical interest for the labora- 

 tory worker. Woodruff finds that Protozoa are rarely introduced 

 from the air; and that Paramecium is not introduced dry, on 

 hay or otherwise. 



Air, water, and hay are all sources from which Protozoa are derived, 

 and increase in importance in the order given. Of these three, however, 



