150 
R. Ruggles Gates. 
of the mutations has been made. In four successive papers (Metz, 
1916a, 1916b, 1918, 1920) a great many interesting facts have been 
recorded. D. vivilis 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 III, 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 
