THE SPLIT OR Y COMB. 13 



per cent. Thus, in the two cases, the proportion of the median element 

 and the range of its variabiUty 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. 



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 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 F^, where the 

 median element is almost or completely repressed (down to 10 to 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 ele- 

 ment and the type of very incomplete dominance. 



We have now to consider how these two types of comb and their fluctu- 

 ations 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 off- 

 spring 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 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. 



We have seen in the last paragraph that the Y comb with only 10 per 

 cent to per cent median element has germ-cells bearing median comb as 

 truly as the Y comb containing 70 per cent to 80 per cent median element, 

 but we have also seen that in the latter case there is an excess of single- 

 combed progeny. We have now to inquire whether, in general, there is a 

 close relation between the proportion of median element in the comb of the 



