The experiments reported show that in theF₁ generation the males are not much deeper in colour than “purerustica,” so that this form may be considered dominant to a great extent over the type; in fact, some heterozygous insects are indistinguishable fromrustica.

The great variation in the buff coloured hybrids is probably due to the effects of modifying genes. The colour of these buff insects as well as that of the others was measured in the tintometer. The construction of distribution curves in the manner previously described enables the segregation of these variable insects to be followed without difficulty.

Segregation is found to take place in theF₂ generation, the purerustica and the black type males segregating from the heterozygous buff coloured insects in an approximately 1:2:1 ratio. Owing to the fact that some heterozygous males are indistinguishable from purerustica, the latter class is found to be too numerous.

The crosses between theF₁ generation and type individuals also show very clear segregation in the males, there being equal numbers of type, and buff coloured or white insects.

The statement that var.rustica is phylogenetically the older race is supported by a consideration of what is known concerning the chemistry of black pigments.

Nevertheless, a repetition of their work shows a ratio of type to melanic in the first broods, very closely approaching the equality expected.

In another set of matings, which produced the second broods, the mortality for various reasons was over 75%. In this case, as in the experiments of Hamling and Harris, the ratio was nearer 3 melanic to 1 type than equality. This suggests that there may have been a selective mortality in favour of the melanic insects, which disturbs the normal ratio.

That a certain constitutional strength seems to accompany many melanic forms has been pointed out by other authors, and attention is here drawn to the fact that this would account more easily than other hypothesis for the rapid replacement of type by melanic varieties.

The factor for this character, calledX, may be considered as a darkening or melanising factor, similar toD in the “agouti-blacks” of Punnett, but rather weaker in its effect. In a heterozygous condition, it converts an agouti rabbit into a steel; and in a homozygous condition, it converts an agouti into a black.

The data show that blacks extracted from steels, and therefore homozygous forX, when mated to agoutis (homozygous forx) give nothing but steel; and moreover, the same blacks when mated to normal blacks or chocolates (aaxx) give nothing but steel.

There is some evidence to show that there may be more than one type of agouti, for two agouti males were used, which when mated to steels gave equal numbers of steel and agouti, but when mated to extracted blacks, one gave nothing but steel, the other both steel and agouti. On this point, further investigations are being made.