212 AN INTRODUCTION TO MODERN GENETICS 



Hartmann has generalized the concept of relative sexuality derived 

 from observations of this sort into a general theory of sex determina- 

 tion. We shall find that in the higher organisms the "sex factors" can 

 vary in potency in the same way as the sexual tendencies discovered by 

 Hartmann and the factors which he postulates to control them. But in 

 the higher forms the variations have nothing to do with the fertiHzation- 

 producing tendencies of the gametes; they concern the degree of reali- 

 zation of morphological sex differentiation in the zygophase. Thus the 

 sexuaHty which is relative in the higher forms is not at all the same sort 

 of thing as the sexuality whose relativeness provides the basis of the 

 theory, and it appears unjustifiable to homologize them. One can, 

 perhaps, draw attention to the fact that in both cases the genetic 

 factors on which the sexuality is based vary in a quantitative manner, 

 but this amounts to no more than pointing out that in both cases the 

 factors are of a kind which can have hypo-hyper-morph allelomorphs, 

 which is true of many factors besides sex factors, and is therefore not 

 very enlightening. 



B. GAMOPHASE SEXUALITY 



We can classify the possible types of gamophase and zygophase 

 sexuality as follows : 



1. Zygophase asexual, gamophase bisexual or with separate sexes 



environmentally determined, e.g. monoecious mosses with bi- 

 sexual gametophyte. 



2. Zygophase asexual, gamophase separately sexed by genetic deter- 



mination, e.g. dioecious mosses. 



3. Zygophase bisexual or with environmental determination of sexes, 



gamophase sex determined embryologically during development 

 in the zygophase, e.g. hermaphrodite animals, higher plants 

 hermaphrodite during the zygophase. 



4. Zygophase separately sexed by a genetic determination, gamophase 



sex determined embryologically as before, e.g. separately sexed 

 animals and higher plants. 



Roughly one may say that what we should ordinarily call sex is 

 associated with the gamophase in i and 2, and with the zygophase in 

 3 and 4. It is usually considered that the evolution of sex determination 

 has been along the series i, 3, 4, with 2 as an offshoot from i. The 



