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THE AMERICAN NATURALIST [Vol.XLIV 



while DR X DD gives none in these grades. A similar 

 result is found in the apparently blending character of 

 nostril height. May not segregation occur in the cases 

 of apparent blending! Does not true blending occur only 

 in complex characters such as stature and skeletal weight? 

 Of such character complexes the component units may 

 still segregate. 



Finally, the fact of imperfect dominance leads to an 

 explanation of many puzzling cases of apparent failure of 

 inheritance. Some years ago I bought two tailless cocks 

 A and B, of which B was said to be the son of A. They 

 were certainly very similar in appearance. I mated A to 

 some tailed hens and their offspring were tailed. The 

 next year I mated these hybrids with each other and the 

 females with their father. Were taillessness recessive, 

 as I suspected, one-fourth of the progeny of the first 

 mating and half of the progeny of the second should have 

 been tailless. Actually there was produced not one 

 tailless bird. One would apparently have been justified 

 in concluding that taillessness is not an inheritable condi- 

 tion. But when the second tailless cock was mated with 

 the tailless hybrids approximately half of the offspring 

 were tailless; and such tailless offspring bred inter se 

 have in turn produced a large proportion of tailless 

 progeny. This whole case at first seemed inexplicable to 

 me, as it did to Professor Bateson, to whom I related it, 

 but it receives a satisfactory interpretation on the theory 

 of imperfect dominance. For rumplessness, or rather 

 an inhibitor of tail growth, is dominant over its absence. 

 But with cock A this inhibitor is so impotent that in the 

 heterozygote, at least, it does not make itself felt and 

 even in the second hybrid generation the duplex de- 

 terminer fails to activate fully. I say fully, for there 

 was a trace of activity. At least fifteen per cent, of the 

 offspring were recorded as having a small uropygium and 

 in many of the adults the back appeared shortened and 

 bent and the tail drooped instead of standing erect. De- 

 spite these evidences of the activity of the inhibitor, the 



