C. DOBELL 147 



attained'. Conjugation has never occurred in this line'-. It is con- 

 cluded that the " cycle " of Calkins (§ 28) " is probably an artificial one 

 which is brought about by the subjection of the race to an environment 

 which is not suitable for its prolonged existence " (Woodruff, 1911 d). 



40. Woodruff' and Baitsell (1911, 1911 ct) have found that "when 

 P. aurel'ia is bred on a varied culture medium, or on a constant culture 

 medium of beef extract, cycles do not occur, but rhythms (§ 37) persist. 

 It is not possible by constant environmental conditions to eliminate the 

 rhythms." The "constancy" of the environment in these experiments 

 is, however, not above suspicion. 



41. Woodruff (1911, 1911 a) has proved that the excretion products 

 of Paramecium have " a decidedly depressing effect on the rate of repro- 

 duction," and this no doubt accounts to some extent for depression 

 periods in cultures. " These excretion products are essentially specific 

 in their action" (1913a). This result should be compared with those 

 of Enriques (§ 33), Popoff' (§ 34) and Sun (§ 35). The last named 

 investigator has found — like Woodruff — that culture fluid in which 

 Paramecia have been growing is toxic to other Paramecia : but she 

 has made the curious observation that the toxicity is lost if the fluid 

 be boiled. 



42. Baitsell (1912) has succeeded in cultivating several lines of 

 Stylonychia pustulata for a large number of generations. The longest 

 line ended at the 572nd generation (cf Maupas, § 18). " Conjugation 

 never occurred and abnormal or degenerating animals did not appear, 

 but after a gradual decline in the fission rate the culture finally died 

 out." This result was pi-obably not due to " the ending of any definite 

 life-cycle," but to unsuitable cultural conditions. More recently, 

 Baitsell (1914) has come to the same conclusion from studies of 

 Occytricha and Pleurotricha. 



43. If we consider the results obtained by all the investigators 

 mentioned in the preceding paragraphs, it seems highly probable that 

 "depression" or "senescence" inciliates is due to unhealthy surroundings 

 — unsuitable food, toxins produced by the organisms themselves or their 

 cultural companions, etc. — and is not due to any inherent inability to 

 live or frustrated necessity to conjugate. Woodruff's conclusion con- 

 cerning Paramecimn seems equally applicable to all other ciliates : " It 

 is probable that most, if not all, normal individuals have, under suitable 



' Dr Woodiuff informs me that the number of generations now reached (March 20, 

 1914) is 4310. 



'' But see § 101 infra. 



