itself in a variety of intermediate stages. Indeed from a hundred chicks bred in this way it would be possible to pick out a number of individuals and arrange them in an apparently continuous series of gradually increasing pigmentation, with the completely unpigmented at one end and the most deeply pigmented at the other. Nevertheless, the case is one in which complete segregation of the different factors takes {127} [Illustration]place, place, and the apparently continuous series of intermediates is the result of the interaction of the different factors upon one another. The constitution of the F_1 [male] is a ffPpIi, and such a bird produces in equal numbers the four sorts of gametes fPI, fPi, fpI, fpi. The constitution of the F_1 [female] in this case is FfPpIi. Owing to the repulsion between F and I she produces the four kinds of gametes FPi, Fpi, fPI, fpi, and produces them in equal numbers. The result of bringing two such series of gametes together is shown in Fig. 28. Out of the sixteen types of zygote formed one (FfPPii) is homozygous for the pigmentation factor, and does not contain the inhibitor factor. Such a bird is as deeply pigmented as the pure Silky parent. Two, again, contain a single dose of P in the absence of I. These are nearly as dark as the pure Silky. Four zygotes are destitute of P, though they may or may not contain I. These birds are completely devoid of pigment like the Brown Leghorn. The remaining nine zygotes show {128} various combinations of the two factors P and I, being either PPIi, PPII, PpII, or PpIi, and in each of these cases the pigment is more or less intense according to the constitution of the bird. Thus a bird of the constitution PPIi approaches in pigmentation a bird of the constitution Ppii, while a bird of the constitution PpII has but little more pigment than the unpigmented bird. In this way we have seven distinct grades of pigmentation, and the series is further complicated by the fact that these various grades exhibit a rather different amount of pigmentation according as they occur in a male or a female bird, for, generally speaking, the female of a given grade exhibits rather more pigment than the corresponding male. The examination of a number of birds bred in this way might quite well suggest that in this case we were dealing with a character which could break up, as it were, to give a continuous series of intergrading forms between the two extremes. With the constant handling of large numbers it becomes possible to recognise most of the different grades, though even so it is possible to make mistakes. Nevertheless, as breeding tests have amply shown, we are dealing with but two interacting factors which segregate cleanly from one another according to the strict Mendelian rule. The approach to continuity in variation exhibited by the F_2 generation depends upon the fact that these two factors interact upon one another, and to different degrees according