CHROMOSOME STUDIES IN THE DIPTERA 53 



The principal difficulty encountered in the examination of 

 Types IV and V, is that of distinguishing between the sex-chromo- 

 somes and the small autosomes. As shown by the figures, the 

 likeness is so great that at certain stages they cannot be dis- 

 tinguished with certainty. This causes some trouble in late pro- 

 phase and early metaphase, when the autosomes are in the process 

 of contraction, but before and after this the separation may be 

 made readily, so that the difficulty in no way interferes with the 

 identification of the various chromosomes, or with the process of 

 homologizing them with members of other types. 



SUMMARY AND CONCLUSIONS 



The genus Drosophila has numerous species which differ from 

 one another in the number and form of their chromosomes, as 

 well as in external characters. Of the twelve species examined 

 by me five conform to one type in respect to the chromosomes, 

 four conform to another type, and three to three additional types 

 respectively — making five types in all. In each type the chromo- 

 some group is made up of certain different kinds of chromosomes, 

 distinguished by their size, form and behavior. By comparing 

 these types with one another an evolutionary series may be ar- 

 ranged. This series can be explained upon the assumption that 

 four of the types arose from the first by a series of steps, involving, 

 on the one hand, the partition of certain long chromosomes into 

 halves, and on the other hand, the disappearance (by degenera- 

 tion or fusion), in two cases, of a pair of very small m-chromo- 

 somes. It is not likely that the forms studied represent a con- 

 secutive series. In themselves they make a dichotomous series, 

 but in all probability the intermediate steps in this constructed 

 series, if they represent evolutionary stages at all, are merely indi- 

 cations of what has taken place, and do not themselves belong in 

 the positions assigned to them. 



Type I is arbitrarily chosen as the stem from which the others 

 are derived because it includes the greatest range of species, and 

 because it is easier to conceive of an evolution by partition, or by 

 loss, of chromosomes than by their fusion or generation. The 



