In Different Species. 57 



Direct proof of homology of individual mutant characters can 

 hardly be obtained, except by hybridization. On the other hand, 

 it ought to be possible to obtain evidence very nearly as conclusive 

 by the use of series of characters, even without hybridization tests, 

 especially when several related species are being studied. For- 

 tunately, in the case of the drosophilas, deductions from such evidence 

 receive added support from analogy with the one case in which hybri- 

 dization is possible. For example, we may consider the case discussed 

 below, involving the four sex-hnked characters yellow, crossveinless, 

 singed, and forked in virilis and those of the same names in melano- 

 gaster. These characters are morphologically similar in the two 

 species, and in addition they are all sex-linked and their genes come 

 in the same order and at approximately proportional distances on 

 the chromosome maps. This agreement in several respects affords 

 ground for considering them homologous, but the probability is 

 increased still more by analogy with the known cases of homology 

 in D. simulans and melanog aster. Yellow and forked and three 

 characters whose genes occupy intermediate loci in D. simulans 

 have been shown by Sturtevant (1921a) to be homologous to cor- 

 responding characters in melanogaster. The map relations of these 

 are shown in figure 13. To be sure, the two species, melanogaster 

 and simulans, are so nearly identical that resemblances in genetic 

 behavior would be expected to be closer here than in the other cases, 

 but the analogy between the proven relations here and the apparent 

 relations in the other cases can not but add probability to the latter. 



This, however, does not obviate the necessity for extreme caution 

 in considering possible homologies. The dangers in this regard 

 have often been pointed out. It is well known, for instance, that 

 mimic characters may appear in one species, which are somatically 

 identical, or nearly so, but which are due to mutations in different 

 loci or even in different chromosomes. Likewise there is the possi- 

 bility that identical mutations may give different results in different 

 species. These and other possibilities of error emphasize the need 

 of a relatively large amount of data before cases of homology can 

 be considered as established. In the present paper, therefore, we 

 will confine ourselves to a consideration of the trend of the evidence, 

 recognizing the fact that subsequent data may modify the case. 



COMPARISON OF SEX-LINKED CHARACTERS. 



The "Forked and Singed" Series. 



The most extensive series of similar mutant characters in the 

 drosophilas includes those called "forked," "singed," or "stubby." 

 The most prominent characteristic common to these is the modified 

 thoracic bristles. One or more mutants in this category are known in 

 each of the following species: D. melanogaster, D. lirilis, D. willistoni, 



