28 BULLETIN 905, U. S. DEPARTMENT OF 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 theory is that early service tends to result 

 in a preponderance of females, while service late in heat means 

 more males. This theory 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. O. 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, 



