CHROMOSOMES AND EVOLUTION 103 



attachment (Fig. 20), another is a smaller chromo- 

 some with a median spindle attachment, the other 

 two are chromosomes with submedian spindle attach- 

 ments. Within Race B three types of Y-chromo- 

 some have been found, all with submedian spindle 

 attachments (one of these may or may not be the 

 same as one of the types found in Race A). 



It is not possible to state exactly how these different 

 types of Y-chromosome have arisen, but it is clear 

 that rearrangement of sections of the chromosome 

 has taken place, probably involving both duplication 

 of short lengths and actual loss (' deletion ') of certain 

 portions. Since in all races and sub-races the Y 

 appears to be genetically inert the loss of portions 

 of it would not affect the genie balance. 



Drosophila pseudo-obscura thus appears to be 

 breaking up into a number of ' incipient species '. 

 Two of these are already effectively separated by the 

 barrier of hybrid sterility, and each of these is in its 

 turn splitting up into a number of sub-races, which, 

 while not yet inter -sterile, will probably eventually 

 become so. Ordinary gene -mutation appears to have 

 played little part in the separation of all these forms 

 which lends support to the view that it is more 

 important as a factor in species-differentiation than 

 in species-dichotomy. 



When we come to D, miranda we find a species 

 closely related to pseudo-obscura, but separated from 

 it by some remarkable cytological differences. ^^ The 

 mode of sex-determination in miranda is unique 

 among the species of Drosophila in that it involves 

 two pairs of chromosomes, the males being X^ Xg Y, 

 the females Xj Xg X^ Xg. The X^ of miranda clearly 

 corresponds to the X of pseudo-obscura, the sequence 

 of bands in both its arms being similar. The X2 

 chromosomes, of which there are two in the female 

 miranda, but only one in the male, corresponds to 

 the Ilird autosome in pseudo-obscura, and appears 

 to have become involved as part of an entirely novel 



