146 A. FEANKLIN SHULL 



was in a medium which always contained traces of this mixture. 

 Precautions were taken to obviate any error in this regard. In 

 the first place, it seemed likely that the extreme dilution to which 

 any substance peculiar to either environment was subjected would 

 have prevented that substance from exerting any appreciable 

 influence. It must be remembered that the Baltimore pure line 

 was bred for eleven generations under the same treatment as the 

 New York line, before the crosses were made. The first female, 

 and the females of each generation thereafter, were isolated in 

 certainly not more than four drops of water. This was at once 

 diluted to forty or more, or to one-tenth of the original concen- 

 tration. In eleven generations, the concentration of any substance 

 originally present would be only 10"" times the concentration of 

 that substance at the outset. Unless the substance had enor- 

 mous powers of propagation, it would be negligible. But to pre- 

 clude any possibility of error, an amount of water approximately 

 equal to that in which the males were transferred in making the 

 crosses, but containing no rotifers, was taken from each line and 

 placed in the dishes of the other. So that from that time on both 

 of the pure line were being reared in the same mixed environment 

 as were the offspring of their cross. The same precaution was 

 taken in each crossing experiment. 



In both of these experiments (table 35 A and B) the pure line 

 derived from the cross has a decidedly higher proportion of male- 

 producers than did that part of either parent pure line which 

 was bred at the same time. Moreover, it is higher than either of 

 the parent lines produced as a whole, the entire series of genera- 

 tions in the New York line having produced but 11.1 per cent, 

 the Baltimore line 18.5 per cent. It is to be noted that the resting 

 eggs in these crosses were kept at room temperature, whereas 

 many eggs in nature are subjected to continued low temperature. 

 While no experiment of as great duration as a winter season was 

 attempted, some eggs were kept for a brief time, as described in 

 the next experiment, in an ice chest. 



Experiment XXXIV. Two repetitions of the preceding experi- 

 ment were made, except that the resting eggs obtained after cross- 

 ing were kept for some days at a low temperature. Two females 



