104 THE CHROMOSOMES 



mechanism of sex-determination. There is appar- 

 ently a non-random distribution of the sex-chromo- 

 somes at the first meiotic division in the male so that 

 only two kinds of gametes (X^ Xg and Y) are formed. *i 

 Although all the chromosomes of miranda can be 

 homologized in a general way with those ofpseitdo- 

 obscura, a large number (100 at least) of rearrange- 

 ments in the sequence of the genes have taken place. 

 The majority of these are inversions, but some trans- 

 locations of small regions from one chromosome to 

 another have also been established. It is natural 

 that translocations should be rarer in evolution than 

 inversions, since they lead to the production of hypo- 

 and hyper- diploid individuals, whilst inversions do 

 not upset the ' genie balance '. There are indications 

 that the arrangement of the bands in the salivary 

 gland chromosomes of miranda is more similar to that 

 in the A race of pseudo-obscura than to that in the B 

 race. As in the case of the melanogaster-simulans pair 

 of species the differences between the arrangement 

 of the bands in the microchromosomes are very great. 



The group of species related to Drosophila affinis 

 includes at least seven species ; they are fairly closely 

 related to the pseudo-obscura forms, but will not 

 interbreed with them ; the chromosomal diflferences 

 have not been fully worked out as yet. 



The species D. montium contains two definite races 

 (A and B) and there are probably more. The two 

 known races differ in regard to the IVth pair of auto- 

 somes which are V-shaped in Race A and rod-shaped 

 in Race B ; it seems clear that one arm of the IVth 

 chromosome has been completely lost in Race B ; 

 possibly it is genetically inert in Race A.^'' The two 

 races of montium can clearly be regarded as ' incipient 

 species \ 



To sum up : we can say that the process of species- 

 dichotomy appears to be going on at a considerable 

 rate in Drosophila, so that each species is breaking 

 up into a number of * incipient species * distinguished 



