Parallel Mutations. 53 



of the mutations has been made. In four successive papers (Metz, 

 1916a, 1916b, 1918, 1920) a great many interesting facts have been 

 recorded. D. virilis is known (Metz, 1914, 1916c, 1916d) to have 

 six pairs of chromosomes, including one pair of small or m- 

 chromosomes, whereas D. melanogaster has 4 pairs. In the latter 

 species it has been shown that the 200 or more mutations all fall 

 into the corresponding four groups. In D. virilis Metz (1920) has 

 recently reported a total of 27 mutations, including 14 which are 

 sex-linked (Group I), 3 in Group II, 4 in Group HI, 3 in Group IV, 

 and 3 in Group V. No mutations have yet been found corresponding 

 to the small pair of chromosomes, but the late appearance of 

 mutations from such small chromosomes is to be anticipated on the 

 laws of chance, and the two known members of the corresponding 

 group in D. melanogaster were not discovered until a very large 

 number of mutations were known. 



In an earlier paper (Metz, 1918) dealing with eight sex-linked 

 characters in D. virilis, half of them were found to correspond 

 closely to mutations of D. melanogaster. These four were yellow 

 body-colour, magenta eye-colour, glazed eye-surface, and forked 

 bristles on the thorax. The yellow mutation is slightly darker than 

 in the case of D. melanogaster, but this is to be expected if the 

 changes correspond, since the normal virilis is a darker species. In 

 the forked mutation " the morphological correspondence is complete 

 even to the smallest detail." Yellow and forked are not only sex- 

 linked mutations, and hence in the X chromosomes, in both species, 

 but their linkage relations are shown to be similar, so that they 

 occupy corresponding positions in the X chromosome, yellow being 

 terminal and forked about 55 units from it as measured by the 

 percentage of crossing over. There are indications that the actual 

 amount of crossing over is greater in D. virilis. 



Confluent, referring to the veins on the wings, is another 

 mutation which has its counterpart in D. melanogaster. It is a 

 dominant, is not sex-linked, and when present in a homozygous 

 condition has a lethal effect, as shown by the ratios of confluent in 

 crosses and by the absence of homozygous individuals, though there 

 is still a possibility that some may occur in D. melanogaster. Black 

 body and morula-eye are other non-sex-linked mutations occurring 

 in both species. 



The six pairs of chromosomes in D. virilis have apparently 

 been derived by transverse segmentation of two long boomerang- 

 shaped pairs of chromosomes in D. melanogaster, the other two 



