3S8 PERMANENT HYBRIDS 



mechanism replaces an earlier one. No doubt also environmentally 

 or developmentally determined systems can readily develop into 

 genetic systems with new sex chromosomes. The beginnings of 

 such systems are present everywhere. 



In Zea Mays, which normally has unisexual flowers on different 

 parts of the same plant, dioecious stocks have been produced in 

 two ways, one giving a homozygous male, the other a homozygous 

 female. In each case a combination of two mutant genes is 

 necessary, one of them, however, being homozygous in both sexes 

 (R. A. Emerson, 1932). 



In Lebistes new gene combinations have been obtained in which 

 the sex differentiation is determined by a single pair of allelomorphs 

 in autosomes. Two such combinations have been selected, one 

 giving males, the other females, homozygous. In both, the homo- 

 zygous sex has been reversed to give the new heterozygous sex, 

 while the old heterozygous sex type with its Y chromosome has 

 been eliminated (Winge, 1934). 



Hence, we see how the sex determining mechanism can be 

 reversed in the course of evolution. We see also how nearly 

 related fishes, as well as birds and mammals, come to have opposite 

 systems. Similarly in Drosophila simulans (Sturtevant, 1929), 

 D. virilis (Lebedeff, 1934) and Lymantria dispar (Goldschmidt, 

 1934) genes or gene-combinations or cytoplasmic variables are 

 known which partially reverse one or both of the sexes. But in 

 these organisms complete and fertile reversal of the homozygous 

 sex seems to be impossible. Apparently the Y chromosome can 

 be replaced only when it either differs from the X in the minimum 

 degree or in the maximum degree when, as we shall see, it has 

 become entirely inert, i.e., in the simplest and in the most advanced 

 types of sex chromosome differentiation. In the intermediate 

 condition there is something essential in the Y that is not in the X ; 

 but at either extreme there is nothing. 



Evolutionary changes of other kinds can be inferred from the 

 comparison of related species. Of these the commonest is the 

 complete disappearance of the Y chromosome in groups some 

 species of which have an XY — XX system {e.g., in Hepaticae and 

 Orthoptera). The most remarkable of these is found in Drosophila. 



