LIFE CYCLE OF HYDATINA SENTA St 
erations (figs. 5 and 6). The simultaneous decrease of vigor and 
of the proportion of male-producers could be readily ‘explained’ 
in part by assuming that the two phenomena are correlated. 
Opposed to this view is the fact that inbreeding in Hydatina 
seems on the whole, to result in a diminution of vigor, whereas 
the proportion of male-producers appears on the average to be 
unaltered by inbreeding. Such could not be the case if any true 
correlation between the two phenomena existed. A further objec- 
tion to the view that vigor and the proportion of male-producers 
are correlated is found in the results of experiments (3 and 4) 
in which lines derived rom females that remained long in the fer- 
tilized egg are compared with lines derived from females that 
hatched quickly from the egg. If the duration of the egg stage 
is inversely proportional to vigor, as one might expect, a correla- 
tion between vigor and the proportion of male-producers would 
result in a lower percentage of male-producers in parthenogenetic 
lines derived from late hatching females. This appears not to be 
the case. The decrease in vigor due to inbreeding may be ac- 
counted for if we assume that vigor is due to the degree of hetero- 
zygosis of the individuals, as has been found to be the case with 
corn (G. H. Shull, 708). But this assumption will not explain 
the decrease of vigor with long-continued parthenogenesis (figs. 
5 and 6). 
We have seen that the internal nature of a parthenogenetic line 
of Hydatina is subject to some degree of change dependent on the 
age of the line, but that initial differences also exist among differ- 
ent lines. We may thus conceive the internal nature, as far as it 
concerns the life cycle, to be composed of at least two parts: 
First, the genotypic constitution, determined at the moment of 
fertilization, and, barring irregularities partaking of the nature of 
mutations, remaining constant through many generations; and 
second, a changeable element which is probably to be included in 
Woltereck’s reaction-norm. We may either add to these a third 
element which causes the great fluctuations (‘waves’) in the pro- 
portion of male-producers, or assume that the reaction-norm is 
itself very variable. 
