The Life-History of Hypotrichous Infusoria. 6oi 



IV. DISCUSSION OF THE DATA OF THE CULTURES. 

 I. Rhythmical and Cyclical Variation in the Rate of Division. 



One has but to glance at the plotted curves of the various cul- 

 tures (Diagrams I to VI) to see that all the species of Infusoria 

 studied pass through periods of greater and less dividing activity 

 when subjected to a stable environment. These periods, upon 

 analysis, are resolved into two kinds: First, the short, more or 

 less rhythmical fluctuations in the fission-rate which I shall 

 refer to as "rhythms"; and second, the long downward trend of 

 the cultures (especially prominent in the Oxytricha A-culture) 

 from their beginning to end, or, in the case of Oxytricha A, from 

 its start to its recovery by stimulation at about the 250th genera- 

 tion, and again from this point through the second long downward 

 sweep which ended with its extinction. This second type of 

 change of fission-rate I regard as the "cycle." I am satisfied that 

 these two kinds of variation are due to different causes. I believe 

 the rhythms to be somewhat superficial in character and due in 

 part to slight variations in the environment, the most important 

 of which is change in temperature. This belief is based on the 

 remarkable agreement which obtains between the rhythms and 

 the fluctuations in temperature. In Diagram VII there is plotted 

 a section of the culture-curves of all the four cultures which were 

 carried on simultaneously, and above them the temperature 

 curve. The agreement is seen to be more marked in the Oxy- 

 tricha cultures; in the Pleurotricha series, the similarity, while 

 not as striking, is too exact to be a mere coincidence, and serves 

 to emphasize the fact that while temperature does influence the 

 rate of multiplication, it is not the most important element among 

 the factors which cause fluctuations in the rate. It is only natural 

 that temperature variations should aff^ect the division-rate, if not 

 directly, at least indirectly through the eff'ect on the multiplica- 

 tion of bacteria and therefore upon the food-supply, and this has 

 been shown to be the case by Maupas and Calkins. 



I believed there was another and more fundamental factor 

 underlying the rhythms, and with this in mind I took still 

 greater precautions to have the environment as nearly constant 

 as possible in the more recent culture of Gastrostyla steinii 

 (see note, p. 600). From the results of this series when plotted 

 in periods of ten days (Diagram V), it would seem that my idea 



