198 HEREDITY AND SEX 



This is an assumption, but perhaps not an unreasonable 

 one. Let me illustrate why I think it is not unreason- 

 able. If the highest possible point of productivity 

 is a complex condition due to a large number of things 

 that must be present, then any change is more likely 

 to be downward, since at the beginning the high-water 

 mark had been reached. In time casual selection would 

 be likely to pick out a poor combination — if this hap- 

 pened once the likelihood of return would be small. 



As we have seen (Chapter I) Maupas found in a 

 number of protozoa that if he picked out an individual 

 (after each two divisions) to become the progenitor 

 of the next generation, the rate of division after a 

 time slowed down. The individuals became weaker 

 and finally the race died out. Calkins repeated the 

 experiments with paramoecium on a larger scale and 

 obtained similar results. The question arose whether 

 the results were not due to the hay infusion lacking 

 certain chemical substances that in time produced an 

 injurious effect. Calkins tested this by transferring 

 his weakened strains to different culture media. The 

 result was that the race was restored to more than 

 its original vigor. But very soon degeneration again 

 set in. A new medium again restored vigor to some 

 degree, but only for a short time, and finally the 

 oldest culture died out in the 742d generation. It 

 was evident, therefore, that if the slackened rate of 

 division and other evidences of degeneration were in 

 part due to the medium, yet some of the effects 

 produced were permanent and could not be effaced by 

 a return to a more normal medium. Then came 

 Woodruff's experiments. He kept his paramoecia on 



