148 W. J. MOENKHAUS 
It is an interesting fact that it is possible to develop a strain with 
a high female ratio much more easily and pronouncedly than a 
male strain. I have repeatedly tried to hold the sex-ratio to or 
below that of unity but without success. Not infrequently pairs 
will throw a predominance of males but it has not been possible 
to hold them there. The best I have ever been able to do is to 
hold it considerably below that of the normal, but never as low as 
unity. On the other hand, it is relatively easy to select in the 
direction of females even to the extent of 1 to 2. 
It should be observed that in the breeding of these strains the 
most rigorous inbreeding was practiced. It might, therefore, 
be that the difficulty of selecting for a low female ratio results 
from the possibility that inbreeding tends toward the elimination 
of the males. My extensive experience in inbreeding these crea- 
tures, however, does not bear out this explanation. Furthermore, 
in the sixth generation of the high female strain it,was possible 
in two generations to reduce this ratio to near unity notwith- 
standing that the same rigorous inbreeding was continued. 
4. Relative influence of male and female in determining the sex-ratio 
Having thus produced two strains showing a decided difference 
in the sex-ratio of their offspring I wished to determine two further 
points. First, whether the maternal or the paternal elements had 
an equal share in the control of this ratio, and second, whether this 
ratio was determined in the process of fertilization. To this end 
reciprocal crosses were made between the two strains and the pro- 
portion of the sexes in the offspring ascertained. Three experi- 
ments were performed in the following manner. From among 
a brood of each of the two strains a large number of individuals 
were taken. Before sexual maturity a number of males and females 
were isolated, while the remainder were allowed to reproduce. The 
latter gave a control for each of the strains. The isolated virgin 
females of one strain were mated with the males of the other. 
Each experiment thus consisted of four multiple matings. (1) A 
number of brothers and sisters belonging to the male strain. This 
furnished a control for the male strain. (2) A number of brothers 
and sisters belonging to the female strain. This furnished a control 
