136 INTRODUCTION TO SEXUAL PHYSIOLOGY 



on any considerable scale and affecting many individuals is the 

 exception and not the rule, and it would be unreasonable in the 

 present state of our knowledge to expect a theory of sex-deter- 

 mination to account fully for every known case.^ 



Other Theories of Sex-Determination. — Innumerable theories 

 of sex-determination have been put forward in the past, but 

 none of these has been satisfactory or applicable to more than 

 a limited number of facts. According to one theory the right 

 ovary produces ova of one sex and the left of the other (Dawson), 

 but this view is negatived at once for birds, which have only one 

 ovary, and for those animals in which one ovary has been removed. 

 Another theory affirms that the sex of the future offspring depends 

 upon the time of copulation during oestrus, and this again has 

 been disproved for special cases. The effects of feeding have 

 often been made the basis for theories of sex-determination, and 

 while it is possible that they may account in part for exceptional 

 cases of sex change, no hypothesis so far put forward has been 

 found to be of general application. On the other hand, it is 

 evident, in the light of many of the ascertained facts referred to 

 above, that the other theories of sex-determination must be 

 rejected, though certain of them have not been without value, 

 since they have stimulated inquiry, and led to an accumulation 

 of evidence bearing on more than one problem of biology. 



' In connection with Cloklschmidt's theory of inteisexuality in tlie 

 gypsy moth, it must be remembered that in the Lepidoptera the male is 

 homozygous for sex and the female is hetei-ozygous (seep. 120). Con- 

 sequently the male has two X-chromosomes in the body cells and the 

 female one X, or the exact reverse of what occurs in most other animals 

 as far as is known. 



