392 PERMANENT HYBRIDS 



• 

 conditioned, as Muller pointed out (1918) by its crossing-over 



being completely suppressed. This inertness may be shown by the 



fact that the entire Y can be lost with no effect on the fly, apart 



from sterility, in Drosophila melanogaster (Bridges, 1916 ; Stern, 



1929), and in Banasa and Metapodius (Wilson, 1905, 1928). It 



may also be shown by the inertness of fragments that are derived 



from the Y. As many as six may be found in Metapodius without 



apparent effect. Similar fragments are found in the Tettigidae 



(Robertson, 1916) and probably in Blaps lusitanica (Nonidez, 



1921). 



In Drosophila, for a reason that is not apparent, the inertness 

 extends to the pairing segments of X and Y (Muller and Painter, 

 1932). Possibly this change is general, for inert fragments probably 

 derived from X are found in Alydus (Reuter, 1930) and elsewhere 

 (cf. Table 16). 



(viii) The Evolution of Sex Chromosomes. According to these 

 different systems of crossing-over restriction, and the progressive 

 changes that are conditioned by them, we may trace the develop- 

 ment of the sex chromosomes, from their uniform beginning of 

 minimum differentiation to their uniform end of the obliteration of 

 the Y, by various different routes (cf. Diagram, Fig. 115). These 

 steps represent the possible direct series of evolutionary changes. 

 Such series may be interrupted at any point, as we have seen, by 

 the establishment of a new mechanism in autosomes which will 

 usually begin again at the first step. 



The evidence shows that the differentiation of sex chromosomes 

 evolves by the following steps : — 



1. Origin of a gene-pair (the sex differentials) whose segregation 

 determines the opposite sexes. 



2. Genotypic suppression of crossing-over in the region of the 

 sex differentials. 



3. Structural and gene changes in this region, making it into a 

 differential segment. 



4. Special gene changes preserved which render the differential 

 segment of Y inert and give that of X a new internal balance to 

 compensate for the inertness of Y (Muller, 1932). 



5. Further structural changes may, first, include autosomes in 



