1926] Bahcocl-Lesley : Chromosomes and Taxonomic E elation ships 333 



McCluiig (1908), on the basis of observations on many genera of 

 Orthoptera, says, 



Merely as a result of the study I have made of the germ cells I would have 

 classified these insects into two groups, one having a complex of twenty-three 

 chromosomes and the other of thirty-three. On the other hand, many taxo- 

 nomists, from careful and minute examination of the external anatomy of these 

 same species, had agreed in ijlaeing them into family groups whicli they call 

 the Acrididae and Locustidae. 



McClung (1917) has made an especially thorough study of the genera 

 Hesperotettix and Mermiria, and has had the benefit of the cooperation 

 of experts on the classification of the Orthoptera, with similar results. 



Metz (1914, 1916) has shown that the Drosophilidae have rather 

 similar chromosomes and that the species form several groups on the 

 basis of their cytological characteristics. Metz and Lancefield (1922) 

 state that the 13 species belonging to class A, of which D. meJanogaster 

 is an example, are scattered throughout the genus. The Drosophilidae 

 are of especial interest from the standpoint of cytology and taxonomy, 

 since something is known of the arrangement of genes within the 

 chromosomes of several species, and it is therefore possible to com- 

 pare the chromosomes from a genetical as well as a purely morpho- 

 logical viewpoint. Sturtevant (1921) says, "44 recessive mutant 

 genes in 41 loci of I), melanogaster and 12 recessive mutant genes 

 of D. simulmis (in 12 loci) are also recessive in melanogaster-simulams 

 hybrids." Some of these genes are found in each of the 4 chro- 

 mosomes indicating that "The data from D. simulans show what 

 was suggested by the other results and by much cytological data, that 

 the constitution of a chromosome may be essentially the same in two 

 different species. ' ' Both of these species belong to type A cijtologically 

 (Metz and Moses, 1923) and are closely related taxonomically. The 

 evidence from D. ohscura and D. wiUistoni, on the other hand, shows 

 that the chromosomes which one would naturally suppose to be 

 identical on the basis of purely cytological criteria are not the same 

 genetically, since Metz and Lancefield (1922) state: "In the two 

 species having V-shaped X chromosomes, then, yellow and scute are 

 'located' near the middle of the chromosome map, while in melano- 

 gaster with its short rod-like X chromosome, yellow and scute are on 

 one end." Metz and Moses (1923) emphasize the importance of 

 genetical evidence in any attempt to evaluate the significance of 

 similarities or differences of a cytological type. 



Lists of chromosome numbers also contain what appear to be many 

 flagrant exceptions to the view that the species of a genus will be cyto- 



