Wilson — Agouti-black Colour in Rabbits. 597 



more be said •witli regard to X', X, and Y. It might be suggested 

 that X' and X have each a blackening effect, and that Y has a lightening or 

 yellowing ; but if the blackening effect of X' produces a black rabbit wlien 

 concurrent with the yellowing effect of Y, why does the further addition 

 of the blackening effect of X make the previously black rabbit lighter or 

 yellower ? Every suggestion thought of is met with this anomaly in one form 

 or another ; and eventually appeal may have to bo made to the physiological 

 chemist. 



Our solution being thus presumably sound, we must now consider how it 

 differs from Professor Puunett's. His assumption is that there are three 

 dominant factors and tlieir " absences " operating in the case, and that one 

 called D cannot enter a gamete unless accompanied by another called JE. 

 His hypothesis is as follows : — 



" In the set of experiments into wliich ,$ 28 and his descendants enter we 

 are dealing essentially with three separate factors : 



" A, the ' agouti ' factor which turns black into agouti, and tortoise into 

 yellow. 



" H, a factor for the extension of the melanic pigment which turns yellow 

 into agouti and tortoise into black. 



" D, a factor of wliich the effect is to produce a deepening in the melanic 

 pigment. 



" The effect produced by B depends (1) upon whether this factor is present 

 in a homozygous or in a heterozygous condition, and (2) upon whether the 

 animal is homozygous or heterozygous for E. Tlie addition of one dose of D 

 to an agouti which is homozygous for £! turns it into an agouti-black, while 

 the addition of a second dose results in a full black. If however the agouti 

 is heterozygous for E, the addition of either one or two doses of J) produces 

 the same visible effect, viz., a full black. The presence of i) in a black makes 

 no difference to the appearance of the animal."' 



With regard to this hypothesis two remarks may he made. According 

 to it, the following constitutions correspond to the colours set down above 

 them : — 



The first remark is that this solution also meets with the anomaly met 

 with in the new solution when considering whether the functions of the 



' Journal of Genetics, vol ii, p. 227. 

 SCIENT. PECO. E.D.S., VOL, XHI., NO. XXXVII. 4 T 



