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 follows: Absence of single comb is allelomorphic 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 all grades between these extremes, including minute papillæ 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 F1. Yet they provide us with all the data for an explanation. Median comb of the Minorca dominates over no median of the Polish, and so in F1 we have the median element represented. But, on the well-known principle of imperfection of dominance in F1, 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.
B. VARIABILITY OF THE Y COMB AND INHERITANCE OF THE VARIATIONS.
As already stated, the proportions of the median and the lateral elements in the Y comb are very variable; the median element may, indeed, constitute anywhere from 100 per cent to 0 per cent of the entire comb. Even full brothers and sisters show this variability. Thus the offspring of No. 13 ♀ Single-comb Minorca and No. 3 ♂ Polish have the median element of the Y comb ranging from 0 per cent to 70 per cent of the whole comb. Notwithstanding this variability of the median element in any family there is a difference in the average and the range of variability in families where different races are employed. Thus the offspring of two Polish × Minorca crosses show an average of 46 per cent of the median element in the comb; the Houdan × Minorca cross gives combs with 60 per cent of the median element; and in the combs of the offspring of two Houdan × White Leghorn crosses there is, on the average, 71 per cent of the median element. The Houdan × Dark Brahma (pea comb) gives combs with an average of 87 per cent median element and the Polish × Rose-comb Minorca cross gives 89 per cent median. The rose-combed hens used in this last cross were heterozygous, having single comb recessive; consequently they produced also chicks with typical Y combs. Such had, on the average, only 59 per cent of the median element and were thus in striking contrast with the slightly split rose combs. In the case of the partially split rose combs the median element ranged from 60 per cent to 100 per cent of the whole length of the comb; but in the split single combs the range is from 0 to 100 per cent. Thus, in the two cases, the proportion of the median element and the range of its variability differ greatly.
Also, in generations subsequent to the first, the Y comb exhibits this same variability. We have already seen that the progeny of the Y-combed offspring of any generation may be compared with those of any other, and so we may mass together the progeny of all hybrid generations so long as they are derived from the same ancestral pure races.
Fig A.—The frequency of the different forms of Y comb, each form being based on the percentage of the median element of the Y comb to the entire length of comb.
In inquiring into the meaning of this variability we must first construct the polygon of frequency of the various grades of median element. This is plotted in fig. A, which is a composite whose elements are, however, quite like the total curve. There is one empirical mode at 70 per cent and another at 0 per cent. The smaller mode at 50 per cent is, I suspect, due to the tendency to estimate in round numbers, and may be, in this discussion, neglected. From this polygon we draw the conclusions, first, that the median element in the Y comb tends to dominate strongly over the absence of this element, as 7:3, and, second, that dominance is rarely complete. Yet there is an important number of cases, even in F1, where the median element is almost or completely repressed (down to 10 to 0 per cent of the whole) and the comb consists of two high and long lateral elements—the "cup comb" of Darwin. There are, then, in the offspring of a median-combed and a non-median-combed parent, two types with few intergrades—the type of slightly incomplete dominance of the median element and the type of very incomplete dominance.
We have now to consider how these two types of comb and their fluctuations behave in heredity. When two parents having each combs of the 70 per cent or 80 per cent median type are mated, their offspring belong to the three categories of I, Y, and "no-median" comb, but the relative frequency of these three categories is not close to the ideal of 25 per cent, 50 per cent, and 25 per cent, respectively. For there is actually in 336 offspring a marked excess of the I comb, 36 per cent, 44 per cent, and 20 per cent, respectively, resulting. When, on the other hand, two parents having each combs of the 10 per cent and 0 per cent types are mated their offspring are of the same three categories and the proportions actually found in 241 offspring (28 per cent, 47 per cent, 25 per cent) closely approximate the ideal. It is clear, then, that even the cup comb, without visible median element, has such an element in its germ-cells and is totally different in its hereditary behavior from the Polish comb, in which the median element is absent, not only from the soma, but also from the germ-cells.