Internal Characteristics and Size 121 



of Oxytricha that larger body sizes prevailed when- 

 ever fission was slower. Popoff ('07) stated that the 

 opposite relationship held true in Stylonychia. In some 

 of Jennings's cultures of Paramecium the size in- 

 creased; in some it decreased, with slower fission. It 

 must be obvious that the result depended not only 

 upon the species observed, but also upon the condi- 

 tions which led to the modification of fission rate. Few 

 observers of living protozoa believe that changes of 

 fission rate, whether they occur in rhythms or not, 

 are wholly independent of external conditions. As has 

 been shown extensively in an earlier chapter, all sorts 

 of circumstances may retard the fission rates, as com- 

 pared with the rates prevailing under standard cir- 

 cumstances. Some of these conditions encourage as- 

 similation or increase of size, and some do not. 



Summary. All the events that were above discussed 

 might be regarded as due to internal factors in the 

 regulation of size. But unicellular organisms, of all 

 organisms, are at the mercy of environments. The 

 events are controllable to a very large extent, if not 

 entirely. If the visible reorganizations are responses 

 to external conditions, then the size changes may be 

 either in response to the reorganizations or directly in 

 response to the same external conditions. It is a 

 healthy point of view to realize that internal modifica- 

 tions are in response to many kinds of conditions ; some 

 can be seen and some cannot be seen. Who then shall 

 say whether a modified size or fission rate is the result 

 of an unseen and unknown internal change or of a 

 perhaps known external change? But both views 

 reduce to an identity when a measured modification is 

 simply correlated with a measured change, whether it 

 be external or internal; and as little as possible is said 

 about cause and effect. 



