412 Scientific Proceedings, Royal Dublin Society. 



preferring the absence of the pedestal as tlie cause which brings the bust to 

 tlie ground and overlooking the action of gravity. 



As might be expected, the presence and absence theory, when used for 

 analytical purposes, falls into the error of overlooking real recessives. Being 

 frequently hidden, they are not always, readily identified and connected with 

 their proper dominant. Then, when the dominants in a case under analysis 

 are already allotted their own absences as their only possible recessives and 

 one or more recessives turn up without their connexion with their dominants 

 being identified, so many more factors are introduced beyond the number 

 which the case can hold. One such case will be sufBcieut by way of 

 example. 



Mr. G. 0. Hurst made a long series of experiments with rabbits.' He 

 started witli what seemed to be only three kinds, namely, grey, black, and 

 albino. But the albinos were found to be of two kinds, and thus he really 

 started with four pure kinds. When greys and blacks were mated, the first 

 crosses were all grey, and the second crosses greys and blacks in the ratio 

 3 : 1. Thus grey seemed dominant to black. When greys were mated with 

 one albino, the first crosses wei-e all grey, and the second crosses greys and 

 albinos in the ratio 3:1. Tims, grey seemed dominant to albino also. But 

 when grey was mated with the other albino, wliile the first crosses were grey, 

 the second crosses were greys, blacks, and albinos in the ratio 9:3:4. Thus, 

 while the first two cases indicated that there was only a pair of determinants 

 concerned in each, the last case showed that there were more. In this last 

 case there are presumably two pairs of determinants concerned ; but one of 

 the second-cross groups of three is indistinguishable from the group of one. 

 Three of the albinos form one of the two groups of three, and the fourth 

 albino forms the group of one. If this assumption results in a disagreement 

 with the facts, it can be abandoned. Let us find what each determinant stands 

 for. Write down the four groups by name, with the non-committal formula 

 and unknown symbols below : — 



At first sight it would appear as if the two dominants were albino and 

 black ; but, since albino is carried by the last group, it must be recessive, 

 and, since the same character is carried by the third group, it must be 



' See Journal of the Linncean Society, Zoology, toI. xxix, p. 283. 



