igo8.J JENNINGS— HEREDITY IN PROTOZOA. 411 



III. GROWTH IN PARAMECIUM. 



One significant fact was noted in the breeding experiments 

 described in a previous section. Whenever a large and small speci- 

 men (belonging to a given group) were isolated at the same time, 

 the large specimen as a rule divided first. Often at the end of 

 forty-eight hours the large specimen had produced eight or sixteen 

 progeny, while the small specimen had either not divided at all, or 

 had produced but a single pair. 



This suggests that the differences in size may be largely matters 

 of growth; that the small specimens may be 3'oung ones, and that 

 the variations shown in the frequency polygons may be largely 

 growth differences. It is clear that a study of growth in Para- 

 mecium is imperative before intelligent work can be done with 

 variation. The subject of growth in the Protozoa is an interesting 

 one in itself, so that this study will be made as thorough as possible 

 for its own sake, as well as for the light it throws on variation. 



Growth was studied by three different methods: (i) By obser- 

 vation of abnormal specimens bearing localized appendages, noting 

 the changes in position during growth; (2) by following the changes 

 of form and size in living specimens; (3) by a statistical examina- 

 tion of the dimensions of individuals of known age. 



The observations on growth in abnormal specimens have been 

 described in my first communication (Jennings, 1908). By obser- 

 vations on the living specimen it is not possible to obtain precise 

 measurements. It will be best therefore to begin our account with 

 the statistical examination, taking up the observations on the living 

 specimens by way of control. 



Effects of Growth ox a Variation Polygon. 



If our suspicion that growth differences make up an important 

 part of the observed variations in size of Paramecium is justified, 

 then cultures rapidl}' multiplying and growing should be more vari- 

 able than those that are stationary. To test whether this is true, 

 two lots were removed from a rather old culture of descendants of 

 D, in which inspection showed that the individuals were not multi- 

 plying rapidly. One of these lots was killed at once, while the other 



