6 Introductory. 



Many cases of species hybrids are known, and some have been 

 studied extensively. Those involving domestic or semidomestic 

 animals, e. g., the mule, ducks, pheasants, etc., are too well known 

 to need more than a mention, as are also those of numerous cultivated 

 plants. Likewise, various kinds of fish hybrids have long been 

 familiar. The work of Federley (1914, 1915) and Harrison and 

 Doncaster (1914) has revealed the fact that hybridization is readily 

 obtained in some families of Lepidoptera. That of de Vries, Davis, 

 Bartlett, and others on Oenothera and of East and others on Nicotiana 

 has demonstrated the same thing for these groups. The list might 

 be continued at length, but the examples given include many of the 

 most thoroughly studied cases. None of the groups known to be 

 favorable for hybridization, however, fulfils the other requirements 

 necessary for a detailed study of chromosome relationships, such as 

 we are considering here. 



The genus Drosophila has long been known to include species that 

 are excellent for breeding purposes, and the pioneer work of Morgan 

 and others on D. melanogaster indicated that this species, at least, 

 mutates frequently enough to provide ample material for genetic 

 analysis. Consequently, when it was found (Metz, 1914) that other 

 species of Drosophila differed from melanogaster and from one another 

 in respect to their chromosomes, it was recognized that the material 

 offered unusual possibilities for a study of chromosome relationships 

 and chromosome evolution. The only uncertainty was with regard 

 to the possibility of hybridization. If favorable in that respect, all 

 of the above requirements would be fulfilled. With this in mind, 

 an extensive series of tests was carried out cooperatively by Dr. A. H. 

 Sturtevant and one of us (Metz) during 1914 and 1915, involving 

 most of the species obtainable in the United States and Cuba, save 

 those that were not at all amenable to laboratory treatment. The 

 results of these tests were all negative, however. This presented a 

 serious difficulty, since it indicated that if such a comparative study 

 were undertaken with this material it would be necessary to make 

 separate genetic studies of selected species and then compare the 

 results. 



However, an excellent foundation for such an investigation was 

 provided by the well-known observations on D. melanogaster; and 

 since a study of this kind promised to be of interest in several addi- 

 tional respects, it was undertaken. 



It should be noted at this point that Dr. Sturtevant has recently 

 succeeded in hybridizing D. melanogaster and D. simulans (Sturte- 

 vant, 1920) with very interesting results. For the purposes outlined 

 above, however, the possibilities here are limited. The two species 

 are almost identical and appear to have identical chromosome 

 groups, and in addition the Fi hybrids are sterile, so that only mutant 



