104 GENETIC VARIATIONS 



they show unfavorable symptoms of many kinds. The prog- 

 ress of these changes is well shown by plotting a curve of the 

 frequency of division of the animals (Figure 17). This curve 

 of the rate of reproduction gradually "runs down hill," 



10 20 30 40 50 60 70 80 90 100 110 120 150 140 150 160 170 180 

 Figure ly. Partly smoothed curve showing the gradual decline in 

 the rate of reproduction in Stylonychia pustulata, when subjected to 

 a high temperature (28 to 32 degrees). The vertical scale shows the 

 mean number of fissions per day; the horizontal scale shows the num- 

 ber of days from the beginning of the high temperature. Constructed 

 from the data of Middleton, 1918. 



though with various fluctuations. There is on the whole a 

 rather steady downward progress, sometimes to a final cessa- 

 tion of reproduction, and death of all the organisms. The 

 slowing of reproduction is accompanied by degenerative 

 changes in the other functions above mentioned. The degen- 

 erative changes are not perceptible till many generations have 

 elapsed, and many more generations are required for the slow 

 descent to deep depression or death. 



As to the exact nature of the changes that are occurring, 

 there have been differences of opinion. Some have believed 



