42 AN ANALYSIS OF THE EFFECT OF SELECTION. 
race, and not produced by the cross of angoraXshort. 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. Sporrep GuINnEA-Pias 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. EnauisH Rassits. 
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 F; English, a buck of 
grade 3.75 (only one F; 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 F, 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. 
