128 THE SECOND-CHROMOSOME GROUP 



we come to add to the map the loci of the genes as yet unpublished, 

 another such cluster will appear at the left end of the chromosome 

 also. These clusters at the ends may be looked upon, not as due to 

 the genes being here actually nearer together, but to the probability 

 that at the ends crossing-over is relatively less frequent than in the 

 middle part of the chromosomes. The bearing of the information as 

 to the relative frequency of double crossing-over on the conclusion just 

 stated is discussed in the section on " Purple." 



To the reader who is not especially concerned with the localization 

 of the genes, we should like to call attention to other subjects of very 

 general interest, such as the discussion of modifiers in the sections 

 on purple, the creams, star, and the second-chromosome modifier of the 

 third-chromosome character, dichaete. Another topic of general inter- 

 est is that of autosomal and balanced lethals discussed in the sections 

 on truncate, streak, confluent, star, dachs-lethal, and lethal Ila. A 

 third topic of interest is that of variations in the amount of crossing- 

 over due to specific genes and to such factors as age and temperature 

 that are discussed in the sections on purple, dachs-lethal, and in the 

 summary dealing with the cross-over variations Cm, and C// r . 



SPECK (s p ). 



(Text-figures 73 a and b, 75 6, and plate 5, figures 1 and 4) 



ORIGIN AND STOCK OF SPECK. 



In the course of the early work upon Drosophila in the Columbia Zoo- 

 logical Laboratory a selection experiment was carried out by Morgan 

 upon a race of wild flies that had showed variation in the extent and dark- 

 ness of the shield or trident pattern upon the thorax. In the fourth gen- 

 eration of selection for a race " without" such a trident, there appeared 

 (March 1910) a few individuals with a tiny black speck (plate 5, fig. 4) 

 at the juncture of each wing with the thorax (Morgan, 1910). At first 

 the breeding results obtained with this character were irregular (Mor- 

 gan, 1910). Some of this irregularity may have been due to non-virgin 

 females (24-hour females were used) and to the practice of using mass 

 cultures, though probably more was due to difficulty of classification 

 before familiarity with the characteristics of the mutation had been 

 acquired. 



A stock pure for the character was obtained, but was set aside in 

 order that more time might be given to the study of the sex-linked 

 eye-color white which had appeared in April 1910. About a year 

 after this (May 1911) it was found that a stock of flies with a dark 

 body-color called "olive" was pure for a character which was taken to be 

 the same as speck (text-figs. 73 a and 73 6). Accordingly, the first and 

 simpler speck stock was discarded and the olive stock was retained. 

 There is some uncertainty with regard to this "olive" stock, but it seems 



