No. 60S] MULTIPLE FACTORS IN MICE AND RATS 479 



Size inheritance studies have not been recently made 

 with mammals of known ancestry and of approximately 

 pure races. This fact greatly diminishes the value of the 

 results obtained even though they represent work of the 

 most painstaking sort. For this reason the writer started 

 last January a series of experiments on size inheritance 

 in pure races of rabbits. Polish rabbits are being used for 

 the small parent and Flemish giant rabbits for the large 

 parent. It is hoped by a careful study of variation within 

 the pure races to understand more clearly the method of 

 inheritance in the hybrid generations to be produced by 

 later experiments. 



To summarize the work with inoculable tumors, one 

 may say that it presents a type of inheritance not expli- 

 cable on an hypothesis of blending inheritance or of a 

 single variable gene. All the results may, on the other 

 hand, be satisfactorily explained by supposing that the 

 nature of the host tissue and its reaction to the implant 

 depend upon a complex of mendelizing factors. 



Conclusion 



The fact that three genetically distinct types of spot- 

 ting exist in mice ; that segregation of the degree of spot- 

 ting occurs in both rats and mice; that segregation of 

 minute quantitative characters like the "blaze" spotting 

 in mice, and the pattern of the ''mutant" rats occurs; and 

 finally that the composition and reaction of epithelial 

 tissue. in mice depends upon a complex of mendelizing 

 factors, all indicate that in mammals the multiple factor 

 hypothesis is steadily being strengthened as a scientific 

 theory and a practical principle of great interest and im- 

 portance. 



BIBLIOGRAPHY 



Castle, W. E. 



1909. Studies on inheritance in rabbits. Carnesio Inst, of Wash., 

 Publ. No. 114. 

 Castle, W. E., and John C. Phillips. 



