574 



THE AMERICAN NATURALIST [Vol. XLVIII 



hooded strain which had undergone plus selection (and which so 

 contained nearly all of the plus modifiers originally present in 

 either the self or the hooded ancestors) . Thus it was to be ex- 

 pected that, just as a cross of self with the minus race gave F 2 

 hooded rats darker than the original minus strain, so a cross of 

 wild or Irish rats with hoodeds resulting from the plus selection 

 would give F 2 hooded rats lighter than those of the plus strain. 

 This result was actually obtained. It was fatal to the idea that 

 the difference between the P, strain of hooded rats and the F 2 

 hoodeds was due to contamination of the allelomorph for hooded 

 with that for self, since such contamination should have resulted 

 in F 2 hooded rats darker than those of P x , not lighter. For wild 

 and Irish rats are both much more extensively pigmented than 

 hoodeds even of the plus strain. 



The change in hoodedness from P! to F 2 was therefore due 

 to recombinations of the modifying factors wherein the two 

 strains differed. That many such modifiers were concerned is 

 indicated by the evenly distributed variability of the F 2 hoodeds 

 and the fact that very few were as extreme as the hooded grand- 

 parents. The same fact is brought out in a cross of the minus 

 with the plus race; here no clear-cut ratios were obtainable, the 

 classification into different genotypes being rendered impossible 

 by the multiplicity of factors (no one of which was hypostatic 

 as in the other crosses). Of course, this knowledge of so many 

 factors being concerned in the crosses helps our interpretation 

 of the selection results decidedly, for the more numerous are the 

 factors concerned, the longer would it be possible to continue an 

 effective selection on the progeny of the hybrids, and the orig- 

 inal hooded rats of the selection experiments were admittedly in 

 all likelihood descended from just such hybrids. The exact num- 

 ber and effect of the different factors can not be determined from 

 Castle and Phillip's data, since to do this very special crosses 

 must be made and individual pedigrees kept. Selection experi- 

 ments can be of little value so long as there are factors for which 

 the individuals may be heterozygous, unless these factors can be 



Of course, it is quite possible that in the course of these long- 

 continued experiments mutations affecting the hoodedness occa- 

 sionally happened to arise, especially since it seems likely that 

 this character is dependent upon an unusually large number of 

 genes, for then, as a matter of mere chance, any mutation which 



