DEPARTMENT OF EXPERIMENTAL EVOLUTION. 127 



nearing completion. Much time has been devoted to securing and preparing 

 material for study during the winter. This has involved the dissection and 

 cytological preparation of hundreds of specimens. Many preparations have 

 been completed and studied, with numerous interesting results. In addition 

 many thousands of flies have been bred and studied for genetic purposes. A 

 more detailed account of these studies follows. 



"Most of the cytological as well as the breeding work has centered in the 

 genus Drosophila, because it offers an unusual combination of favorable char- 

 acteristics, most of the species being comparatively easy to breed and exhibiting 

 striking specific differences in their chromosomes. In addition to the 12 

 species reported in the above-mentioned paper, several others have recently 

 been studied. All but one of these fall into the general scheme proposed for 

 the others. One species, however, is remarkably unlike the rest cytologically, 

 and is of exceptional interest because it is favorable for a study of the matura- 

 tion and prematuration stages in the male, a condition greatly desired because 

 of its bearing on previous breeding experiments in Drosophila. 



"The cytological work on Drosophila falls into three divisions: One is con- 

 cerned chiefly with the relation between the chromosome groups of different 

 species. Thus far about 18 species have been studied with the following 

 results: Each species has been found to possess a characteristic group of 

 chromosomes. These chromosomes are arranged in pairs, which differ from one 

 another in size, form, and behavior in such a way that they may be readily 

 distinguished. Thus the individual chromosome pairs in any species may be 

 separately identified and followed. This fact has made it possible to compare 

 the different specific chromosome groups with one another, chromosome by 

 chromosome, with the result that individual chromosome pairs have been 

 homologized throughout the series, and that the different groups have been 

 found to make up what appears to be a definite evolutionary series, in which 

 the various steps can be clearly followed. The results demonstrate beyond 

 any doubt the real existence of the phenomenon of chromosome pairing, an 

 association of homologous maternal and paternal chromosomes; and they 

 add very substantially to the accumulating evidence of the individuality and 

 continuity of the chromosomes — all of which facts have a very direct bearing 

 on the relation between chromosomes and heredity. 



"The second phase of the cytological work depends entirely on the breeding 

 results — i. e., whether or not species-hybrids are secured. Since the crosses 

 have not yet been obtained, no advance has been made here. This is the most 

 important aim of my Drosophila work, and although only negative results have 

 been given thus far, I am very hopeful of ultimate success, even though consid- 

 erable time and effort may be required. The result depends largely upon how 

 many species can be secured, especially species from distant locahties. The 

 opportunities are unique, without a parallel in plants or animals, and every 

 effort will be made to secure the results, 



"A third line of cytological work on Drosophila centers around a study of the 

 maturation processes, especially in the males. Breeding experiments by 

 Morgan, et at., have indicated that (in one species at least) the relation of the 

 chromosomes during synapsis is different in the two sexes — i. e., homologous 

 chromosomes have no effect on one another in the male, while in the female 

 they have an effect, as shown by the phenomena of "crossing over," etc. I 

 propose to determine, if possible, whether this is attributable to an evident 

 difference in the maturation processes of the two sexes. The work has pro- 

 gressed far enough to show that there are most decided cytological differences, 

 but not far enough to single out a particular one as responsible for the genetic 

 differences. 



