414 Scientific Proceedings^ Royal Dublin Societij. 



is extra ; and B, which was formerly a recessive, is now somt'tliiug common 

 to all. The reason for tliis divergence is that, since, on the presence and 

 absence theory, the only possible alternative to G is its own absence, g, there 

 is no alternative left for blaek but to be pushed out of its place. But this 

 solution is not consistent with the facts of the case, for tlie factor for black- 

 ness was not carried by the original grey parent, nor was it common to all 

 Mr. Hurst's crosses. He showed clearly that it was introduced by one of 

 the albinos. Let us see how the problem is solved on the presence and 

 absence theory. "Applying the presence and absence system to tlie case 

 of the coloTirs of rabbits, tlie first pair of allelomorphs can obviously be 

 represented as — 



Dominant Recessive 



1. Presence of colour (C). Absence of colour {c). 



The second pair we have so far spoken of as the grei/ determiner and tlie 

 black determiner, regarding the two as allelomorphio to each other. But it is 

 equally possible to describe them thus 



2. Grey determiner {G). Absence of ditto {g). 



Then in the case where grey x albino gives in F2 9 grey : 3 black : 4 albino, 

 we simply have to regard B, the black determiner, as common to both 

 parents, and the same numerical result is produced." ' The error arises 

 through failing to realize that g and B are the same. 



Another solution of the same problem on the presence and absence theory 

 miglit also be quoted : — " Agouti," i.e. grey, " was previously known to be a 

 simple dominant to black, i.e., an [agouti is a black rabbit plus an additional 

 greying factor wliich modifies tlie black rabbit 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." With regard to this statement it might be asked : 

 if agouti, since it is dominant to blaek, is a " black rabbit plus an additional 

 greying factor," might it not be regarded equally as an albino plus an 

 additional greying factor, since it also " behaves as a dominant to the albino 

 variety ? " With regard to the above case, it might be pointed out that 

 in a parallel case, viz., that of colour in pigeons, in wliieh the second 

 crosses were 9 black, 3 blue, 4 white, just as the rabbits were 9 agouti, 

 3 black, 4 albino, a parallel- solution was not found as it ought to have 

 been. 



'Bateson, p. 76. ^Pujinett, pp. 48 and 60, 



