INHERITANCE OF PARTICULAR CHARACTERISTICS. 



6 9 



the ' ' narrow ' ' ancestor. Even in the F 2 generation nearly half of the ' ' nar- 

 row and intermediate ' ' were of the intermediate sort. This intermediate 

 form is evidence that dominance is imperfect and segregation is incomplete. 



CEREBRAL HERNIA. 



Cerebral hernia is, as already pointed out, a typical monstrosity. The 

 distribution of its occurrence in crossing is as follows : 



* Excluding one case of egg embryo with cerebral vesicle, 

 t Excluding one egg embryo recorded as doubtful. 



Cerebral hernia is inherited in Mendelian fashion with plain head domi- 

 nant. Nevertheless, many of the plain-headed hybrids have the frontal 

 eminence abnormally high dominance is imperfect. 



CREST. 



The crest is independent of the cerebral hernia (pages 16-18). It is a 

 widespread characteristic among birds, so common that it is not readily 

 thought of as pathological but usually as ornamental. The distribution of 

 its occurrence in crossing is as follows : 



Crest is inherited in Mendelian proportions, and is dominant over crestless 

 head. Even when the Silky is crossed with Callus bankiva its crest is domi- 

 nant (fig. 53). In this case the new characteristic, a positive variant, domi- 

 nates over the ancient one ; but the crest is diminished in the first genera- 

 tion ; dominance is imperfect. 



