REINVIGORATION PRODUCED BY CROSS FERTILIZATION 361 
seventy-five generations and found no increase in sterility or a 
decrease in vigor. Perhaps if these inbreeding experiments can 
be carried on for two to three hundreds of generations there may 
appear an increase in sterility and general debility. In Hydatina 
senta at the 75th parthenogenetic generation there was no notice- 
able decrease of vigor; but much later it gradually appeared as the 
generations increased and the race became older. 
SUMMARY 
1. Two distinct sister parthenogenetic races of Hydatina senta 
characterized by the difference in their rates of reproduction and 
general vigor were developed from one original parthenogenetic 
race under identical external conditions. 
2. Races of Hydatina senta allowed to reproduce parthenoge- 
netically for 384 generations, extending through a period of 
twenty-nine months under identical environments, showed a 
gradual decrease in their rates of reproduction. This was as- 
sumed to signify a decrease in the general constitutional vigor or 
vitality of the race. 
3. Successive inbreedings of the weakened parthenogenetic 
sister races, one to four times, caused a slight increase in their 
reproduction powers. 
4. Reciprocal cross fertilization or cross-breeding of such weak- 
ened parthenogenetic sister races of Hydatina senta caused a 
sudden and very pronounced increase in the reproduction rate 
of the ensuing race. This shows that cross fertilization of the 
two weakened races greatly reinvigorated both races and prob- 
ably restored them to their normal vigor which they possessed 
when they started from the original fertilized egg. 
5. The high reproduction rates of new races of Hydatina senta, 
developed from fertilized eggs which were taken from the same 
general wild culture jar that had been standing at least for 
twenty-nine months is due probably to cross-breeding of different 
races. These different races may have been introduced into the 
culture when it was started or they may have developed since 
from the original race. 
January 2, 1912. 
