302 H. S. JENNINGS 



cetera. Possibly it would be found that under all conditions 

 tending to cause excessive rapidity of fission, conjugation is 

 protective by decreasing this rate. 



The usual relations are found as to the relative variability of 

 the conjugants and non-conjugants. In my paper on rlssorta- 

 tive mating ('11, p. 99), I have shown that the progeny of the 

 conjugants are in this experiment much more variable in size 

 than the progeny of the non-conjugants,. for at least seven genera- 

 tions. Here we need to consider only the variability in fission 

 rate. 



Of the non-conjugants, as we have seen, but sixteen lived 

 through the four days. Their mean rate of fission is 10.813 ± 

 0.233, the standard deviation is 1.379 ± 0.164, and the coeffi- 

 cient of variation is 12.756 ± 1.546. Of the conjugant lines, 

 thirty-six lived through; their mean number of fissions was 

 6.222 ± 0.205, the standard deviation 1.827 ± 0.145, and the 

 coefficient of variation 29.369 ± 2.528. Thus the variation is 

 both absolutely and relatively much greater in the conjugants; 

 if we measure it by the coefficient of variation, the variability in 

 fission rate was more than twice as great in the progeny of the 

 conjugants as in that of the non-conjugants. 



EXPERIMENTS ON PURE STRAINS: CONJUGANTS ALL DESCENDED 

 FROM A SINGLE INDIVIDUAL 



A large number of experiments, some of them extensive and 

 long continued, were undertaken with cultures descended from 

 a single individual. The conditions in such pure strains (or 

 'pure lines', as I have called them in previous papers), are of 

 special interest in some respects, while the results bear likewise 

 upon the same general problems as does the work with wild 

 cultures. 



Race k. These experiments were mostly carried on with the 

 race k, some account of which has been given in my previous 

 papers of 1910 and 1911. This race, belonging to the species 

 Paramecium aurelia, is distinguished by a tendency to conjugate 

 frequently, making it most favorable material for a study of 



