REINVIGORATION PRODUCED BY CROSS FERTILIZATION 359 
same week, rested from two to three weeks, and then were hatched 
in the same period of time, two to five days. Tables 9, 10 and 
11 give the details of the experiments and table 13 shows the 
general results in a summary of these three tables together with 
table 8. From these tables it is seen that the reproduction rate 
of the two crossed races irrespective of the manner of the cross 
approaches very closely to the reproduction rate of race L; 
while the reproduction rates of each of the two races which had 
been inbred were very much lower. This seems to be positive 
proof that in crossing two weak races the resulting race which 
develops from the cross-fertilized egg has its general vigor or vital- 
ity greatly increased. 
The high reproduction rates of the two wild parthenogenetic 
races D and E and of the parthenogenetic races A, B and C at 
their beginning, is probably due to the cross-breeding of different 
races in the same jar. Races A and B were sister races developing 
from the same parthenogenetic mother which originally devel- 
oped from a fertilized egg. If after an interval of about three 
hundred generations each of these two sister races becomes a 
distinct race, as is shown by their different rates of reproduction 
and also by the effects of in- and cross-breeding, it is reasonable 
to suppose that in a general culture Jar standing for two to three 
years many different races are constantly appearing and inbreed- 
ing. Consequently all fertilized eggs taken from such a jar will 
develop into races each having a high rate of reproduction. It 
has been previously stated that races A and B originally devel- 
oped from one fertilized egg taken from a wild culture jar. Race 
C was taken from a wild culture jar and races D and E developed 
from fertilized eggs which were taken from the same general wild 
rotifer culture jar that was made in October of 1908. All of these 
races used came from a general culture of rotifers which was orig- 
inally collected in a certain ditch in Grantwood, New Jersey, in 
October of 1906. 
Shull in his experiments in crossing the New York race with 
the Baltimore race of Hydatina senta caught sight of this same 
fact that cross breeding two races increases the rate of reproduc- 
tion but he was not certain of its validity. His table 37 prob- 
