W. Bateson and E. C. Punnett 201 



afford some indication of them. Where P is not present the bird is 

 always unpigmented, though with regard to I it may be either //, li, 

 or ii. Where / is absent the bird is nearly always fully pigmented 

 whether homozygous or heterozygous for P, though it is probable that 

 chicks recorded as with much pigment may sometimes have been in 

 constitution Ppii. The birds classed as " very fully " pigmented were 

 probably in most cases PPii though sometimes they may have been 

 exceptionally deeply pigmented birds of the constitution Ppii. Where 

 both P and I are present some pigment would appear to be always 

 present though the amount is subject to fluctuation. Thus F^ birds 

 of both sexes (ex Silky % x Brown Leghorn </), and the ^ birds 

 (ex Brown Leghorn $ x Silky (/) are of the constitution PpIi, but 

 in respect of the intensity of their pigmentation they might belong to 

 either of our three classes " slight," " some," or " moderate," and our 

 experience has been that these classes grade very much into one 

 another. Birds with " much " pigmentation are in general either 

 PPII or PPIi, though an occasional bird of the PpIi class might 

 be referred to this group. The class PplI is doubtless to be found 

 among the birds with "faint" or "slight" pigmentation. 



The grade of pigmentation would also appear to differ somewhat in 

 the two sexes, for among birds similarly constituted for these two 

 factors P and / the females are generally a little more pigmented 

 than the males'. 



This case of the Silky pigmentation is interesting in connection 

 with the production of intermediate forms. In an F. family bred from 

 Silky $ X Brown Leghorn (/' all the nine possible zygotic combinations 

 of P and / occur in one or other sex. It would be possible to choose 

 birds of such breeding and to arrange them in a series exhibiting 

 continuous gradation from full pigmentation to none at all. Yet we 

 now know that such a series is due to the interaction of three definite 

 factors (inclusive of the sex factor), and that the continuity in variation 

 manifested is in reality founded upon a discontinuous basis. Moreover 

 we may point out that the mating of partially pigmented males of the 

 constitution PPIIff with partially pigmented females of the constitu- 

 tion PPIiFf would result in the establishing of a race breeding true 

 to an intermediate condition of pigmentation in spite of the underlying 

 discontinuity involved. 



' This fact is interesting in connection with the common experience of fanciers that 

 in black-feathered breeds which have yellow skins, it is easy to obtain males with clear 

 yellow shanks, but the females almost always have some black pigment in the shanks. 



