Varietäten, Descendenz, Hybriden. 5 



by the presence of a "rose" factor; the same considerations apply 

 also to the pea-comb, which is Single plus a pea factor. The terms 

 epistatic and hypostatic, already suggested by Bat eson (Science, 

 Nov. 15, 1907.) are adopted to express the relation subsisting between 

 the rose and the Single comb. Thus the combless, the single-combed 

 and the rose-combed conditions may be looked upon as a cumula- 

 tive series, the factor for Single being dominant to the combless 

 condition but hypostatic to the rose factor. 



This view is illustrated in a new experiment in which the 

 Breda fowl was used. The Breda has ostensibly no comb, though 

 in the male there are two minute papillae Standing one on each 

 side of the middle line which are the rudiments of a comb structure. 

 Experiment has shown that the hens possess the duplicity of which 

 the two papillae of the male are evidence although practically no 

 comb tissue can be detected in the hen. 



The Breda was crossed with a Single, and the resulting F x 

 had a large double comb, formed as two divaricating Singles. The 

 Breda therefore has evidently a factor for duplicity which can 

 split the Single comb, but it is without the Single comb itself. The 

 Breda was also crossed with a rose, and the resulting combs were 

 all duplex roses. Two of these bred together gave duplex and com- 

 mon roses, duplex and common Singles, and Bredas. The fact that 

 Singles appear in the F 2 of such a cross is strongly confirmatory 

 of the view previously expressed as to the relation between rose 

 and Single combs. 



The authors have now concluded their experiments upon the 

 inheritance of the walnut-comb and of its components, the rose, 

 the pea, and the Single, and give a short summary of their conclu- 

 sions. The case is one of simple di-hybridism in which both the 

 factor s affect the same structure, the comb. These factors are roseness 

 (R) which is allelomorphic to its absence (r), and peaness (P), which 

 is also allelomorphic to its absence (p). A walnut-comb is one in 

 which both R and P are present, and such a comb may be homo- 

 zygous or heterozygous for one or both these factors. Theoretically 

 four kinds of walnut-comb and only four are possible, viz. , RRPP, 

 RRPp, RrPP, and RrPp. The authors have met with these four kinds 

 over and over again and with no others. So also there are two kinds 

 of rose and two kinds of pea, viz., those homozygous and those hete- 

 rozygous for the respective dominant factors which they contain. All 

 Single combs, whatever their origin, are of the Constitution rrpp. The 

 results which led to these conclusions are collected together is a table. 



Plumage. In the previous" report it was shown that there are 

 two distinct classes of white fowls — a. those in which white is 

 dominant to colour and b. those in which white is recessive to colour. 



The authors show that the recessive whites are of at least three 

 kinds, viz., 



1. The white birds which have arisen in the course of their 

 experiments (referred to later as the R-Whites). These birds often, 

 but not always, have one or more coloured ticks in their plumage. 



2. The white of the Silky fowl. The adults may be pure white 

 but the down of the chicks often contains some buff. 



3. The White of the Rose-comb Bantam. The adults always have 

 a few coloured ticks, the down of the chicks being pale bluish. 



Each of these three types behaves as a simple recessive to 

 colour, but when the R-white is crossed with the silky white the 

 offspring are all fully coloured. 



