GENETICS PURPLE EYE COLOR DROSOPHILA 305 



changes in the freedom of crossing over. Certain other experi- 

 ments, notably ■ , which give information on the age 



+ pr c sp 



and coincidence changes, have given results that agree better 

 with the first interpretation, though they do not exclude the 

 alternative. The clearest evidence pointing to an internode 

 change is derived from the age-variation experiment made by 

 Plough as a control for his temperature-change cultures 



^ — ^ B. C, 22°, 2 day interval tube control). In this 



experiment the curve for variation in coincidence was the mirror 

 image of the curve of variation in age. The curve of coincidence 

 corresponding to the curve of temperature variation found by 

 Plough seems to be a straight line cutting through the rises and 

 falls of the temperature curve and independent of them. This 

 suggests that the temperature variation is due to a change in a 

 different physiological factor than that involved in the age 

 variation, and that probably it is due to a modification of the 

 coefficient of crossing over of the chromosome itself. 



LITERATURE CITED 



Bridges, Calvin B. 1915 A linkage variation in Drosophila. Jour. Exp. 



Zool., vol. 19, pp. 1-19. 

 Bridges, C. B., and A. H. Sturtevant 1914 A new gene in the second chromo- 

 some of Drosophila and some considerations on differential viability. 



Biol. Bull., vol. 26, pp. 205-212. 

 MacDowell, E. C. 1915 Bristle inheritance in Drosophila. Jour. Exp. Zool., 



vol. 19, pp. 61-98. 

 Morgan, T. H. 1912 Complete linkage in the second chromosome of the 



male of Drosophila. Science, vol. 36, pp. 4-6. 

 Morgan, T. H. 1912 A modification of the sex-ratio and of other ratios in 



Drosophila through linkage. Zeit. f. I. A. u V., Bd. 7, 343-345 

 Morgan, T. H. 1911 No crossing over in the m.ale of Drosophila of genes in 



the second and third pairs of chromosomes. Biol. Bull., vol. 26, 



pp. 195-204. 

 Mtjller, H. G. 1916 The mechanism of crossing over. Am. Nat., vol. 50, 



pp. 193-221, 284-305, 350-366, 421-434. 

 Plough, H. H. 1917 The effect of temperature on crossing over in Drosophila. 



Jour. Exp. Zool., vol. 24, pp. 147-209. 

 Weinstein, Alexander 1918 Conicidence of crossing over in Drosophila 



melanogaster (ampelophila). Genetics, vol. 3, pp. 135-172. 



