28 BULLETIN" 905, U. S. DEPARTMENT OP AGRICULTURE. 
Thus, while it appears very doubtful whether sex can ever be con- 
trolled in any exact way, the possibility of varying the sex ratio 
must be recognized. The results of even this very limited degree of 
control of sex determination, however, have been disappointing to 
date. Slight differences in the sex ratio among young born in differ- 
ent times of the year have often been published, but the results are 
so inconsistent that no general conclusion can be drawn. Another 
common theory is that a better nutritive condition of the dam favors 
the production of females. Great quantities of statistics have been 
gathered on this subject, but the evidence indicates an excess of 
males as often as of females under favorable conditions. Again it is 
believed by some that the sex of the more vigorous parent prepond- 
erates among the young, and by others the reverse is held. The 
two antagonistic theories seem to have just as much and as little 
support. The same is true of the theories which connect the relative 
or absolute ages of the parents with the sex of the offspring. A 
belief which is especially common among cattle breeders is that the 
time of services after the beginning of heat determines sex. The 
most common form of the theoiy is that early service tends to result 
in a preponderance of females, while service late in heat means 
more males. This theoiy has been most thoroughly tested by Prof. 
Pearl in data obtained from Maine farmers. His earlier data seemed 
to support the theory, but after adequate numbers had been obtained 
no significant differences remained. 
The view that sex, or at least sex ratio, can be modified by con- 
trol of such factors as those listed above has been urged most force- 
fully in recent years by Dr. Oscar Riddle on the basis of experi- 
ments begun by Professor C. 0. Whitman with various wild species 
of pigeons and doves. In these experiments Whitman and Riddle 
found an excess of males under conditions tending toward heightened 
vigor and of females under the reverse conditions. In tame pigeons, 
Cole and Kirkpatrick have shown that the sexes of squabs of the 
same clutch are distributed wholly at random, indicating the lack 
of any external control over sex. The departures from a random 
distribution found by Whitman and Riddle were not very great and 
their significance still seems to be an open question. 
A theory of sex determination which deserves mention only because 
of the frequency with which it is advanced is that sperm cells from 
one testicle produce males, from the other females. An alternative 
theory has it that it is one ovary which produces males, the other 
females. These theories are very easy to test by experiments in 
which one testicle or ovary is removed. Such experiments have 
been performed on a large scale with hogs and rats without any 
effect on the sex ratio. 
