POLYDACTYLISM. 



27 



But a more critical examination of the parentages of the 5 pens shows 

 that they are not comparable. In matings 6 to 14 of table 20 the cock is 

 almost certainly a dominant in respect to toes; whereas the cocks in table 

 21 are probably heterozygous. The heterogygous state determines two 

 things : the imperfect nature of the extra-toe and a relative deficiency in 

 the offspring of the higher toe-numbers. In our results we can not say that 

 one of these things is the cause of the other, as Castle does ; they are, rather, 

 in all probability, due to a common cause. I think Castle's paper may 

 justly be criticized for not giving sufficient data concerning the ancestry 

 of the individual mothers used. Without such data the paper can not be 

 said satisfactorily to demonstrate his conclusion. 



Table 22. Summary of observed toe-numhers in offspring, percentages. 



To summarize: "Potencj^, " as measured by dominance of the extra- 

 toed condition, is inherited, in the Houdan crosses at least. There is some 

 evidence, derived from "pure-bred" Silkies, that differences in the degree 

 of development of the extra-toes are inherited. But the average condition 

 of the toes in the offspring of second or later generation hybrids can not 

 be used as evidence of inlieritance of the degree of parental development 

 of the toes, since these are dependent on the same basal cause, namely, 

 the hidden gametic constitution of the parents. Despite the obscuration 

 of imperfect dominance, polydactyUsm in poultry proves itself to be a unit- 

 character that segregates. 



