INHERITANCE OF SEX AND RELATED PHENOMENA 215 
birds, and various fantastic, ornamental, and combative characters, 
usually confined to the male. Much historical interest attaches to 
secondary sexual characters because of the attention directed to them 
by Darwin’s theory of sexual selection. With that we have no particular 
concern in the present chapter, but shall only consider the inheritance of 
them in one form as it is related to the inheritance of sex. 
In the foregoing discussion no particular reference has been made to 
sex-factors, because after all so little is known concerning them. In 
some cases we have found sex accompanied by differences in chromosome 
content, one sex containing an equal pair of chromosomes which are 
represented in the opposite sex by an unequal pair, in another case the 
difference in sex appears to depend upon whether the individual possesses 
the haploid or diploid number of chromosomes. We have also noted 
that there are two different types of sex-inheritance, one in which the 
male is heterozygous and the other in which the female is heterozygous. 
It is only fair to conclude, therefore, that until more light is thrown 
upon these matters, the assumption that sex-determination depends 
upon a sex-factor rests on a rather slender basis. The experimental 
evidence, it is true, is strictly analogous to certain types of Mendelian 
inheritance, and an interpretation of the sex-factor may be given which 
does no violence to our ideas of the complexity of sex-differences. Thus 
it has been shown by ample evidence that the color of eyes in Drosophila 
depends upon the codperation of a number of different factors; we can- 
not say definitely how many, but mutational changes have indicated 
that over twenty-five different loci have something to do with the re- 
actions concerned in pigment production in the eye. Yet in spite of 
this fact the presence of a single factor may make all the difference 
between a red eye and a white eye. Similarly the sex-factor may act 
in conjunction with a whole series of other factors, yet the difference 
dependent upon its presence in the homozygous or heterozygous con- 
dition may make all the difference between the two sexes. At least in 
one form, however, we have even more definite evidence of the presence 
of a definite sex-factor. Shull has shown in Lychnis that where males 
are expected, hermaphrodite mutants occasionally appear. If we offer 
the same explanation for the occurrence of these mutants as we have 
offered for the occurrence of mutations in Drosophila, a change in a 
single locus in the hereditary system, then the appearance of these 
hermaphrodites might be offered as almost conclusive evidence of 
the sex-determining action of a single sex-factor in this particular case. 
The evidence here becomes even stronger when we consider the fact that 
this particular type of change is reversible. Some additional light may 
be thrown upon this question by the consideration of secondary sexual 
characters as related to the inheritance of sex, although thus far the 
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