TRANSITIONS IN MORPHOLOGICAL CHARACTERS 101 



two cultures had become approximately the same. Again no 

 transition to the humped type had occurred. These facts point 

 to the conclusion that individual vai'iations in parental size have 

 little direct influence upon the size and character of the offspring. 

 These internal factors having proved themselves inadequate to 

 produce the mutation-like change, a number of experiments 

 were conducted in which external agents — temperature, food, 

 and cultural media — were variously modified, in the hope that 

 among these the requisite factor or factors would be found. 

 The following experiments may be instanced. 



Influence of lowered temperature 



An individual, the fourth member of the twenty- third genera- 

 tion of D 2, was transferred on December 5, after two days of 

 life at room temperature (20°C.), into a temperature varying 

 between 8 and 12°C. Two well developed young were born 

 directh^ after the change, but no more were born until after five 

 days. This as well as the slow rate of development through a 

 period of about a month during which but four generations were 

 produced may undoubtedly be ascribed to the lowered tempera- 

 ture. Not only the rate of reproduction but the activity of the 

 rotifers was greatly decreased. The history of this series is 

 recorded in chart 2. 



On January 1, a failure of the heating plant resulted in the 

 complete freezing of these cultures which were consequently 

 discontinued. Freezing, however, does not necessarily kill A. 

 amphora, for some which were frozen in solid ice were slowly 

 thawed and continued to live. Moreover, the experiment dem- 

 onstrated that a reduction in temperature does not cause the 

 mutation-like change. 



Influence of a fluctuating temperature 



Still further to test the influence of temperature change, an 

 individual, sister to the one used in the preceding experiment, 

 was allowed to reproduce under repeated fluctuations of tempera- 

 ture. A room temperature of 20°C. was followed by cold (5°) 

 which was then raised again to 20°, and finally maintained at 27 



