34 Structural Characters: Animals [CH. 



43. Polydactylism occurred in a Guinea-pig, the off- 

 spring of normal parents, and ran an irregular course in 

 its subsequent descent (Castle, 49). 



CAT. 



44. The abbreviated tail of the Manx Cat is a dominant 

 (more or less imperfect) to the normal tail (see Anthony, 2 ; 

 Hind, 151; Davenport, 98 ; Kennel, 166,0). 



45. Polydactylism is almost certainly dominant; but, 

 as in other types, irregularities doubtless occur. 



FOWLS. 



For the study of heredity Fowls are especially well 

 suited. In addition to their many colour-characteristics the 

 various breeds present a great range and variety of struc- 

 tural features. 



Among the long series of offspring which hens of the 

 more fertile breeds produce, the descent of these charac- 

 teristics can be watched in families of ample length. The 

 chief papers dealing with Fowls are R.E.C. (19-22); Hurst 

 (156); Davenport (101). The following is a list of the 

 principal facts already elicited as to the behaviour of these 

 structural features but much remains to be done. 



46. Various shapes of comb, for example the rose comb 

 and the pea comb, are both dominant to the single comb. 

 The double or longitudinally split condition is also dominant 

 to the unsplit. 



See pp. 61-7. Many of the finer details in regard to the heredity 

 of comb-shapes are not yet clear. The classification of the comb-types in 

 the newly-hatched chickens is generally very easy, but in occasional strains 

 forms intermediate between the pea and the single occur, which may 

 probably be due to subtraction-stages of the pea factor (q.v.\ Some of the 

 singles extracted in f\ from various crosses have lateral " sprigs " as 

 fanciers say. It is not impossible that these irregular processes are due to 

 additional minor factors, but they are subject to so much fluctuation that 

 their descent would be very difficult to trace. The comb of the Silky fowl 

 is a rose, + a trifid element which causes its posterior end to be divided into 

 three irregular points. In F^ from Silky x Single, regular rose combs are 

 produced in those individuals which have the rose factor without this trifid 

 element. 



Attention may be called to the dominance of the median splitting of 

 the comb found in certain breeds, for the facts may have a bearing on the 

 genetics of meristic characters. Splitting of the comb may occur in one of 



