560 



THE AMERICAN NATURALIST [Vol. LIII 



Of the yellows resultiiifr from this mating six when tested 

 proved to be heterozygous for black as well as brown, while four 

 carried brown only; and of the yellow young resulting from 

 these tests four were shown to carry black and brown, while 

 three carried brown only. E(iuality of yellows carrying both 

 black and brown and of yellow carrying brown only was ex- 

 pected in contrast with the observed ratio of 10 carrying black 

 and brown to 7 carrying brown only. 



browii IS ilir ivvcisr i.f ihiit rxiHvti'd l)(vau<c lifdwii (normally 



the expected, and black luis ai)|)fnrt'il in a fnM|ncncy less than 

 one third of the expected. 



There are three theories uhi.-h ini<>lit explain tlu'se facts. 

 (1) Reversal of dominance ivsnltinn- in the dominance of brown 

 over black. This may Ix^ discarded because the hrown young in 



fertilization by m.-ai:. of hrnw ,i uamere. united with brown 



jd^age of the genes 

 es are formed more 

 the above matings 



Of the three theories the last is favored because it affords a 

 satisfactory ex|)lanation of tlie observed facts in harmony with 

 other cases of linkage, and because it is more readily susceptible 



