AN ANALYSIS OF THE EFFECT OF SELECTION. 49 



race X wild) to wild, and the relations of the "mutant" series to the 

 selected series. 



When the plus race was crossed to wild, and F 2 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 hi 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 2 's from this female; and their mean 

 grade was 3.05, as against 3.3 for the remaining F 2 '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. Informr 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 



