176 ELMER ROBERTS 



selection effective. Lutz ('11) decreased the length of veins in 

 the wings of Drosophila. Whether modifying factors were in- 

 volved in this case is not known. All must agree that unit 

 characters vary, but there is some difference of opinion as to 

 whether the germinal cause of variability is modifiers or gametic 

 contamination. 



It might be thought that selection was effective, since the 

 flies placed in the incubator at high temperature showed con- 

 stantly large wings. This, however, may have been due to 

 crossing the vestigial-winged to long-winged flies, introducing 

 factors from the long-winged stock which would have increased 

 the wing size among the vestigial winged segregates in the second 

 generation, if the environment had been favorable. It is also 

 conceivable that there was a factorial contamination which 

 would show fully only under high temperature. 



Why the males were affected more easily by temperature than 

 the females is not known. Investigation is now in progress with 

 some hope of finding evidence on this point, 



VI. LITERATURE CITED 



Castle, W. E. 1915 Mr. MuUer on the constancy of Mendelian factors. Am. 

 Nat., vol. 49, pp. 37-42. 



1916 Genetics and eugenics. Harvard Univ. Press. Cambridge. 



1917 Piebald rats and multiple factors. Am. Nat., vol. 51, pp. 

 102-114. 



Castle, W. E., and Alexander Forbes 1906 Heredity of hair-length in 

 guinea-pigs and its bearing on the theory of pure gametes. Carnegie 

 Instit. Wash., Publ. No. 49. 



Castle, W. E., and John C. Phillips 1914 Piebald rats and selection. Car- 

 negie Instit. Wash., Publ. No. 195. 



Castle, W. E., and Sewall Wright 1916 Studies of inheritance in guinea- 

 pigs and rats. Carnegie Instit. Wash., Publ. No. 241. 



Cu^not, L. 1904 L'heredite de la pigmentation chez les souris. (3me note.) 

 Arch, de Zool. exper. et gen. (Scr. 4), T. 2, notes et Revue, pp. 45-46. 



Dexter, John B. 1914 The analysis of a case of continuous variation in Droso- 

 phila by a study of its linkage relations. Am. Nat., vol. 48, pp. 

 712-758. 



Ewing, H. E. 1914 a Pure line inheritance and parthenogenesis. Biol. Bull., 

 vol. 26, pp. 25-35. 



1914 b Notes on regression in a pure line of plant lice. Biol. Bull., 

 vol. 27, pp. 164-168. 



