AN ANALYSIS OF THE EFFECT OF SELECTION. 49 
race Xwild) to wild, and the relations of the ‘‘mutant” series to the 
selected series. 
When the plus race was crossed to wild, and F, hoodeds were ex- 
tracted, it was found that in these extracted animals the mean grade 
was lighter (less ‘‘plus’’) than that of their selected grandparents. 
This, as MacDowell pointed out, is the expectation on the multiple- 
factor view. But Castle now states that when these extracted hoodeds 
are again crossed to wild, and hooded is extracted once more, the 
twice-extracted hoodeds are about midway in mean grade between 
their extracted grandparents and the uncrossed plus race. As he says, 
the wild race might have been expected to bring these animals still 
farther away from the plus race if modifying factors were involved. 
Evidently it is very important that we know as much as possible about 
the wild rats used in these experiments, in order that we may know 
what they were likely to carry in the way of modifying factors. These 
rats, we are told, all came from the same stock, which was trapped at 
the Bussey Institution in large numbers and was reared for two gen- 
erations in the laboratory. ‘‘In making the second set of crosses, the 
extracted individual has, wherever possible, been crossed with its own 
wild grandparent.’”? An examination of the table given shows that 
not more than 102 of the 256 twice-extracted hoodeds can have been 
produced in this way, unless individuals of the same sex were mated 
together. Just how many of the 102, and which ones, does ‘‘ wherever 
possible” include? How many wild rats were used in the original 
crosses? These questions are important, because it is evident from 
a study of the data that the result emphasized by Castle is due almost 
entirely to the descendants of one original plus-line female; 41 of the 
73 once-extracted hoodeds were F,’s from this female; and their mean 
grade was 3.05, as against 3.3 for the remaining F’s, and 3.17 for the 
generation as a whole. The twice-extracted hoodeds tracing to this 
female were of mean grade 3.47, while those from the other original 
hoodeds were again of approximately grade 3.3. Further data re- 
garding the pedigree and other descendants of the mates of this female 
and of her grandchildren are very much needed. Informe tion regard- 
ing the ancestry of the female herself would also be interesting. 
It should also be pointed out that this case, accepted at its face value, 
is difficult to explain on the view that the hooded-rat results are pro- 
duced solely by variations in the hooded factor itself. On that view 
the changes brought about by crossing are usually referred to con- 
tamination of the factors in the heterozygote. But that interpretation 
leaves entirely unexplained the results of the first cross to wild. If 
the hooded factor is contaminated by its allelomorph, the once- 
extracted hoodeds should be darker than their grandparents, whereas 
in reality they are lighter, as would be expected on the multiple-factor 
