102 THE CHROMOSOMES 



reached on entirely different grounds, namely that 

 mutations in some regions are less lethal than in 

 others. One may at least tentatively suggest that 

 the regions where the differences between melanogaster 

 and simulans lie were originally duplications which 

 have become more and more unlike one another in 

 the course of the differentiation of the two species. 



The chromosome set of the species in the obscura 

 group is entirely different from that of the melano- 

 gaster group (Fig. 20). It does not appear possible 

 to homologize any large region of the chromosomes 

 of pseudo-obscura with a corresponding region in 

 melanogaster or simulans. So many rearrangements 

 of the genetic material have taken place since the 

 evolutionary separation of these two groups of 

 species that no large region of any chromosome has 

 been unaffected by inversions, duplications, trans- 

 locations, &c. 



The American species pseudo-obscura consists of 

 two main * incipient species ' which have been called 

 Race A and Race B. They can be crossed but their 

 hybrids are sterile *^ (the male hybrids completely so, 

 the female hybrids almost completely). Physiologic- 

 ally Races A and B are thus almost as distinct as 

 melanogaster and simulans ; it is only the absence of 

 definite taxonomic differences which prevents them 

 from being regarded as separate species. They do, 

 however, show difference in rate of development. The 

 main cytological differences between the two races 

 are six inversions (four in the X-chromosome and one 

 each in Chromosomes II and III) ; these inversions 

 are shown in Fig. 20. Each of the two races is sub- 

 divided into sub -races which are also distinguishable 

 by inversions in various chromosomes, but the intra- 

 racial inversions are not identical with the inter-racial 

 ones. The sub-races can also be distinguished by 

 differences in the length of the Y- chromosome. In 

 Race A four types of Y have been discovered. One 

 of these is a chromosome with a subterminal spindle 



