4 A. FRANKLIN SHULL. 



the first daughter of the last generation, divided by one less than 

 the number of generations. 



5. Proportion of cases in which the first daughter did not be- 

 come the parent of the next generation. In all my breeding 

 experiments, whenever a new family was started, the first two 

 daughters were set aside for further breeding, though only one of 

 them was ordinarily used. If the first daughter was apparently 

 healthy and vigorous, she was invariably used. If the second 

 daughter was distinctly more vigorous than the first, the second 

 became the parent of the next generation. Sometimes it was 

 deemed advisable to discard both and use the third, fourth, or 

 fifth daughter. Thus, in a vigorous line, the first daughter 

 should usually be healthy enough for breeding. As vigor de- 

 creased there should be an increasing proportion of cases in which 

 the first daughter was replaced by a later member of the family. 

 I was unconscious of any selection that would have favored other 

 than the first individual in the later lines of each series, for the 

 idea of measuring vigor by this method did not occur to me until 

 the experiments were all finished and the data were being com- 

 piled. 



6. Difficulty of rearing. As the primary purpose of the experi- 

 ments was not to test vigor, but to obtain a large amount of 

 data regarding egg characters, every effort was made to keep the 

 conditions of nutrition, chemical composition of the medium, 

 etc., at the optimum. To this end the food cultures were changed 

 as frequently as seemed advantageous. If the rotifers became 

 less vigorous, they would be more sensitive to changes in the 

 food cultures, and it would be necessary to renew the latter more 

 frequently. 



RESULTS OF THE EXPERIMENTS. 



The first five of these measures of vigor can be expressed in 

 figures. The sixth, though not thus expressible because records 

 were not preserved, is not less valuable. Table I. gives the 

 data under the first five headings. 



The table shows that, notwithstanding fluctuations, there is 

 an evident decrease in the size of family, of both parthenogenetic 

 and sexual females, from the first line of each series to the last 

 line. 



