LIFE-HISTORY OF ROTIFER 239 



reduction in the viability of the eggs, by no retardation in the 

 development, by no reduction in the range or average of egg 

 production, by no reduction in the range or average of the life- 

 cycle, and by no reduction in the size attained by the adult. 



THE EFFECT OF SELECTION 



As we have already seen, there is great variation in the length 

 of life and the number of eggs produced in the different indi- 

 viduals of Proales, the life-cycle ranging from one to eight days, 

 the egg production from one to thirty. 



Are these variations the results of constitutional and hereditary 

 differences, so that, if individuals showing the different rates 

 of egg production and differences in length of life are isolated 

 and allowed to reproduce we shall get stocks differing consist- 

 ently in these respects? 



This question of variation and the inheritance of variation in 

 uniparental reproduction has been much studied, especially 

 since Johannsen ('03), working with self-fertilizing beans, reached 

 the conclusion that selection within the progeny of a single 

 individual is ineffective and that complete regression occurs in 

 the progeny of all individuals showing variation from the mean 

 for the pure line. The same condition has been found not only 

 in self-fertilizing plants, but in plants reproducing vegetatively 

 by tubers, by grafting, and by buds and in animals reproducing 

 asexually by budding (Lashley, in Hydra), by fission (Jennings, 

 in Paramecium), and by parthenogenesis (Ewing, in Aphids). 



By analyzing Johannsen's results statistically, Pearson ('10) 

 found indications that inheritance might exist within the clone, 

 since the correlation ratios diminished as the line of ancestry 

 became more remote. If the genetic constitution of any indi- 

 vidual of a pure line does not depend on its immediate ancestor, 

 but on the type of the line to which they both belong, the 

 coefficient of correlation between any individuals and any genera- 

 tion of their ancestors should be the same as the correlation be- 

 tween the individuals and their immediate ancestors. Later experi- 

 mental work on Protozoa has given data agreeing with the 

 conclusion that selection within a pure line is to a certain extent 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 35, NO. 2 



