234 



THE AMERICAN NATURALIST 



[Vol. LVI 



Discussioisr 



The previous work on the comparative genetical study 

 of different species of Drosophila has been concerned 

 largely with species having different types of chromo- 

 some groups. It has involved mainly the species 

 melanogaster, virilis, funehris, simulans and ohscura. 

 Of these, only melanogaster and simulans have the 

 type of chromosome group with which we are 

 concerned here. The published data on the first four 

 of these species have recently been summarized by 

 Sturtevant (1920) and may be passed over briefly. The 

 data on obscura are in press and our references to them 

 are made with the kind permission of Mr. D. E. Lance- 

 field. 



In melanogaster, virilis, funehris and ohscnra, the evi- 

 dence suggests a tendency on the part of each species to 

 give mutants paralleling those in the others, although 

 the extent of this tendency can not be ascertained ac- 

 curately because of the impossibility of proving the ho- 

 mology of similar characters. In the case of melano- 

 gaster and simulans the parallelism extends to nearly 

 all of the known simulans characters and certain homol- 

 ogies are established by means of hybridization (Sturte- 

 vant, '20, '21a, 2\h). To be sure, the two latter species 

 are almost identical and would be expected to give similar 

 genetical results; but it is of interest to note that there 

 is a close resemblance between the proven cases of par- 

 allel characters in these, and the apparent cases of par- 

 allel characters in the other species. This tends to in- 

 crease the probability of actual parallelism in the latter 

 where a series of linked characters is involved. 



Upon comparing the mutant characters of willistoni 

 with those of the others it is evident that only a few 

 striking cases of resemblance are found Of these the 

 most significant involve the characters yellow and scute. 

 Their morphological resemblances to the yellow and 

 scute in melanogaster have already been noted in the 



