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THE AMERICAN NATURALIST [Vol. XLIX 



greater part of this country. The color of the thorax (dorsal 

 side), in most specimens, is light gray, each hair having a dark 

 blackish brown spot at its t base. These spots are somewhat 

 irregular, and coalesce in certain regions. 



In October, 1914, I collected a number of specimens of D. 

 repleta in the zoological laboratory at Columbia University. 

 About one sixth of these had a lighter color on the thorax than 

 that found in normal flies. The dark spots, while of about the 

 same number and color as usual, were much smaller and only 

 coalesced in a few small regions. Several females of both kinds 

 were isolated and their offspring observed. These females were, 

 in each case, mated with males of their own kind : but they were 

 of unknown age when captured, and several of them had prob- 

 ably already mated with other males. In the tables given here 

 "dark" refers to the normal type; "light," to the new character. 



Light offspring from J and from Q, when mated together, 

 gave 166 lights in the next generation — no darks. Darks from 

 T, mated together, gave 180 darks — no lights. 



On the basis of these results it is probable that the light char- 

 acter is a sex-linked recessive. The two light females, J and Q, 

 had paired with dark males before being captured, since they pro- 

 duced a total of 17 dark offspring: but these darks were all 

 females, showing either that the male-producing sperm of the 

 father carried no dark factor (i. e., that the factor is sex-linked), 

 or that the light character is dominant in the males and recessive 

 in the females. 



Female V, since she produced light sons but no light daughters, 

 must, on either of the above views, have been mated only by a 

 dark male, and she must have been heterozygous for the light 



