One of the earliest sets of experiments demonstrating the interaction of separate factors was that made by the French zoologist Cuenot on the coat colours of mice. It was shown that in certain cases agouti, which is the colour of the ordinary wild grey mouse, behaves as a dominant to the albino variety, _i.e._ the F_2 generation from such a cross consists of agoutis and albinos in the ratio 3 : 1. But in other cases the cross between albino and agouti gave a different result. In the F_1 generation appeared only agoutis as before, but the F_2 generation consisted of three distinct types, viz. agoutis, albinos, _and blacks_. Whence the sudden appearance of the new type? The answer is a simple one. The albino parent was really a black. But it lacked the factor without which the colour is unable to develop, and consequently it remained an albino. If we denote this factor by C, then the constitution of an albino must be cc, while that of a coloured animal may be CC or Cc, according as to whether it breeds true to colour or can {51} throw albinos. Agouti was previously known to be a simple dominant to black, _i.e._ an agouti is a black rabbit plus an additional greying factor which modifies the black into agouti. This factor we will denote by G, and we will use B for the black factor. Our original agouti and albino parents we may therefore regard as in constitution GGCCBB and ggccBB respectively. Both of the parents are homozygous for black. The gametes produced by the two parents are GCB, and gcB, and the constitution of the F_1 animals must be GgCcBB. Being heterozygous for two factors they will produce four kinds of gametes in equal numbers, viz. GCB, GcB, gCB, and gcB. The results of the mating of two such similar series of gametes when the F_1 animals are bred together we may determine by the usual "chessboard" method (Fig. 8). Out of the 16 squares 9 contain both C and G in addition to B. Such animals must be agoutis. Three squares contain C but not G. Such animals must be coloured, but as they do not contain the modifying agouti factor their colour will be black. The remaining four squares do not contain C, and in the absence of this colour-developing factor they must all be albinos. Theory demands that the three classes agouti, black, and albino should appear in F_2 in the ratio 9 : 3 : 4; experiment has shown that these are the only classes that appear, and that the proportions in which they are produced accord closely with the theoretical expectation. Put briefly, then, the explanation {52} of this case is that all the animals are black, and that we are dealing with the presence and absence of two factors, a colour developer (C), and a colour modifier (G), both acting, as it were, upon a substratum of black. The F_2 generation really consists of the four classes agoutis, blacks, albino agoutis, and albino blacks in the ratio 9 : 3 : 3 : 1. But since in the absence of the colour developer C the colour modifier G can produce no