STUDIES ON THE LIFE CYCLE OF PAKAM KCIL'M. 303 



extent if it is to be made to include the life history of the pres- 

 ent culture. I would not suggest that the protoplasm of every 

 wild Paramccium has the potential to attain over twelve hundred 

 generations or more-- undoubtedly there are strong and weak 

 strains among infusoria as among other classes of animals. 

 Again, it is possible that the different races of Paraincciinn 

 which Jennings ('08) has been able to isolate may have a phys- 

 iological as well as a morphological basis of distinction. It may 

 be also that I have been particularly fortunate in my haphazard 

 selection of culture material, so that the proper variations have 

 been available when necessary. It is true that any particular 

 ingredient of the infusion which might be needed would not long 

 be available for the organisms on account of the frequent change 

 of the character of the infusion, and Gregory has recently shown 

 in the case of Tillina, and I have shown in the case of Gastro- 

 styla, that daily stimulation with salts is often more efficacious 

 than an initial stimulation. But Calkins says in regard to the 

 second cycle of his A culture "... in December it was neces- 

 sary to keep them on the stimulant only a day or two to get the 

 desired result. The short treatment at this period sufficed, 

 because they were not allowed to become weakened to the same 

 extent as in the preceding period of depression." It is this factor 

 which has been taken into account in this study, and it is probable 

 that it has contributed largely to the vitality of the culture. 



It must also be kept in mind that a certain amount of judg- 

 ment is exercised in selecting a representative specimen for 

 isolation. By experience one becomes quite familiar with the 

 normal movements, shape, and general appearance of the organ- 

 isms, so that it is possible to select a favorable specimen daily 

 for the continuance of each of the lines. The precaution is 

 nearly always taken to examine the culture again a few hours 

 after the isolations to see how the organisms behave in the fresh 

 culture liquid. If everything does not appear normal, a new 

 set of individuals is isolated from the " stock " (/. c. t from the 

 one, three or seven individuals left after isolation, the number 

 depending on the rate of division during the previous twenty- 

 four hours). Undoubtedly the process practically results in the 

 artificial selection of the organisms which have the highest poten- 



