THE SPLIT OR Y COMB. 11 



particulate inheritance (first hypothesis). Moreover, it must be said that 

 the split comb is obtained also when the Polish-Houdan comb is crossed with 

 a pea comb or a rose comb; and the pea and rose combs can not be said to 

 have "lateral comb absent," as required by the second hypothesis. Con- 

 sequently the second hypothesis is definitely excluded. 



It now remains to decide between the two remaining hypotheses. 

 First of all, it may be said that the perfection with which I and oo combs 

 can be extracted from Y-combed birds indicates that we are here dealing 

 with a case of Mendelian inheritance and, in so far, favors the third hypoth- 

 esis. To accord with the theory of particulate inheritance, of which the 

 first hypothesis is a special case, the two united characters should transmit 

 the mosaic purely; but this they do not do. Hence the third hypothesis 

 is to be preferred to the first. 



Comblessness is a necessary consequence of the second hypothesis 

 and is inexplicable on the first hypothesis. On the third hypothesis it may 

 be accounted for as f oUows : Absence of single comb is aUelomorphic to its 

 presence. The lateral comb is a character common to fowl either with or 

 without the median comb, but it is ordinarily repressed in the birds with 

 single comb and gains a large size when the median element is absent. 

 It is a very variable element. At one extreme it forms the cup comb; 

 at the other there is an absence of any trace of comb. My own records show 

 aU grades between these extremes, including minute papillae on both sides 

 of the head or on one side only, low paired ridges, the butterfly comb, and 

 cup comb shorter than normal. This variability of the lateral element is 

 comparable to the fluctuation in size of the single comb itself, as illustrated 

 by the Single-comb Minorca on the one hand and the Cochin on the other. 

 It is comparable, also, to the fluctuation in the paired part of the Y comb, 

 which we shall consider in the next section, and to the variability of the 

 oo comb as met with in the pens of fanciers. 



The foregoing considerations do not, at first sight, account for the 

 Y comb as seen in Fi. Yet they provide us with all the data for an explana- 

 tion. Median comb of the Minorca dominates over no median of the PoUsh, 

 and so in F^ we have the median element represented. But, on the well- 

 known principle of imperfection of dominance in Fj, the median comb is 

 usually incomplete and, probably for some ontogenetic reason, incomplete 

 only behind. The incompleteness behind permits the development there 

 of the elsewhere repressed lateral comb, and we therefore have the Y comb — 

 evidence at the same time of a repressed lateral-comb Anlage in the single- 

 combed birds and of imperfection of dominance of the single comb in the 

 first hybrid generation. 



