54 Mutations and Evolution. 



pairs remaining unaltered. The evidence from those parallel 

 mutations with similar linkage relationships shows that even the 

 finer elements of the germ-plasm have maintained their relative 

 positions and potentialities from species to species. This indicates 

 a degree of static fixity combined with mutation in single elements 

 which is truly remarkable, indicating that even the finer details of 

 structure in the germ-plasm may maintain their spatial relationships 

 for long periods. A number of the mutations in D. virilis have not 

 appeared in D. melanogaster, though it does not follow that the 

 latter species is incapable of producing them. At present it is quite 

 unknown why one mutation rather than another appears at any 

 given time, though the evidence is strong that each represents an 

 actual transformation of a germinal element, and not merely a loss. 

 Clearly, the conception of parallel or homologous mutations is 

 destined to be a very useful one in germinal analysis and in the 

 study of relationships. In the next chapter we will consider its 

 application to the study of variations in wild species. 



