42 AN ANALYSIS OF THE EFFECT OF SELECTION. 



race, and not produced by the cross of angora X short. The unusual 

 ratios are based on quite small numbers, and the authors admit that 

 there are difficulties in separation of the three classes, apparently 

 due to overlapping. Moreover, we are given the results only in total, 

 not from each mating separately. 



Castle himself has said of this case: " ... a single unit-character 

 is concerned. Crosses in such cases involve no necessary change in 

 the race, but only the continuance within it of two sharply alternative 

 conditions." (Castle, 1911, p. 39.) 



3. SPOTTED GUINEA-PIGS AND RATS. 



The reference given for these cases is MacCurdy and Castle (1907). 

 I am unable to find in that paper any evidence regarding guinea-pigs 

 that bears on the question of contamination. Nothing but selection 

 experiments are reported. There is, so far as I am aware, no evidence 

 of significance in this connection in the more recent literature on 

 spotting in guinea-pigs. 



The evidence referred to from rats is apparently that obtained from 

 crosses between hooded and Irish races. Hooded rats extracted 

 from such crosses had more extensive colored areas than the uncrossed 

 hooded rats. The data given by Castle and Phillips (1914) and ana- 

 lyzed by MacDowell (1916) show that this is true only when the hooded 

 race is a " minus" one. The "plus" hooded race becomes less pig- 

 mented when crossed to Irish (or to self) . MacDowell has shown that 

 these results conform very closely to the expectations based on the 

 multiple-factor view. 



The later evidence on the case of the hooded rat is discussed else- 

 where in this paper. 



4. ENGLISH RABBITS. 



The data for this case are contained in two papers (Castle and 

 Hadley, 1915a, 19156), in each of which the full presentation is made. 

 The spotting of the English rabbit is a dominant character and is 

 somewhat variable. A single heterozygous male, of the grade desig- 

 nated 2, was mated to a number of Belgian hares. 187 English young 

 were produced, of mean grade 2.43, and of these FI English, a buck of 

 grade 3.75 (only one FI English was of higher grade), was then mated 

 to the same Belgian hare females. 189 English young, of mean grade 

 2.92, were produced. 



This case presents no difficulties for the multiple-factor view, since 

 no evidence is given that indicates the original English buck to have 

 been homozygous for all modifying factors, or that prevents us from 

 supposing the Belgian mother of the FI buck to have transmitted more 

 plus modifiers to him than were present in his father. Under the 

 circumstances, it would have been very surprising if the two lots of 

 young had been of the same mean grade. 



