344 DAVID DAY WHITNEY 
resulted from the fourth successive inbreeding of the race. Pre- 
vious to this time race D had been destroyed and consequently 
a new race, H, was started from a fertilized egg from the same wild 
general culture of rotifers from which race D had been started. 
This new race was used as the control. 
Certain obvious parts of tables 9, 10,11 and 12 give the data 
and results of these observations. In these tables it is noticeable 
that the rates of reproduction of races A and B have not risen to 
any marked extent although a slight increase in the rates is appar- 
ent. The conclusion may be safely drawn that successive in- 
breeding of such weak races does not increase their general con- 
stitutional vigor to any considerable degree, even though this 
successive inbreeding is allowed to occur four times, as in race B. 
As the two sister parthenogenetic races have been demon- 
strated to be in a weakened state and this weakness has been 
shown to continue in each race after several successive cross 
fertilizations have taken place it now remains to show what results 
are obtained when these two weakened races are allowed to cross 
breed and reciprocal cross fertilization of the eggs occurs. 
The first series of observations on the crossing of the two races 
A and Bis recorded in table 8. A few females from each of these 
weakened races were put together into one battery jar which 
contained a new food culture. Many males soon appeared and 
after several days eighteen fertilized eggs were taken out and. 
hatched after resting a few days. The rates of reproduction of 
seventeen of these females were determined and compared with 
the reproduction rates of ten different females of the new race D. 
The average reproduction rate of these seventeen females was 
much higher than either of the average reproduction rates of the 
two parent races A and B which have been compared with the 
reproduction rate of race D in tables 1 to 4. In fact 1t even ap- 
proached closely to the reproduction rate of race D. If the records 
of three of these seventeen females which were probably not the 
result of a cross fertilization are eliminated the two average repro- 
duction rates are much closer. This great increase in the repro- 
duction rate and its close approximation to that of the control 
was assumed to be due to a reinvigoration caused by cross fertili- 
zation. 
