54 • CHARLES W. METZ 



direction of evolution, however, is purely a matter of opinion and 

 may quite readily be reversed. 



As far as the characters of the insects themselves a^^e concerned, 

 they throw little light on the question. The various species are, 

 for the most part, quite different from one another, with the ex- 

 ception of two, and these have similar chromosomes (Type I). 

 From the taxonomic standpoint the genus has never received ade- 

 quate attention from entomologists, and consequently there is 

 little clue as to the relationship of species. 



DISCUSSION 



A detailed discussion of the various questions suggested by this 

 study would be out of place at this time, but it may be profitable 

 to call attention briefly to some phenomena appearing in these 

 flies, which do not present themselves so conspicuously in other 

 animals. 



Individuality of the chromosomes 



In practically all the species used in this study the metaphase 

 plates stand out as clearly as diagrams, and in most cases there 

 cannot be the slightest doubt as to the identity of any particular 

 kind of chromosomes. Especially is this true of such types as II 

 and III, in which most of the chromosomes are short. And even 

 in the other types where more long chromosomes are present there 

 is no difficulty in identifying the different sorts except at certain 

 stages. Consequently, after studying scores of preparations from 

 larvae, pupae and adults, and finding that in each of these the 

 same particular kinds of chromosomes appear over and over 

 again with the greatest regularity, both in the somatic and germi- 

 nal tissues; and then after noting that each of these kinds has its 

 own particular method of behavior in each cell division, I can find 

 little reason to doubt that every chromosome has a specific char- 

 acter of its own, and that its individuality persists from one gener- 

 ation to another. Upon no other assumption can I explain the 

 exact repetition of identical chromosome groups in certain species, 

 and the appearance in other species of groups so related to one 



