EVIDENCE OF MULTIPLE FACTORS IN MICE 

 AND RATS 



C. C. LITTLE 

 Harvard Medical School 



The object of this paper is to record certain data on the 

 inheritance of two complex characters and analyze these 

 data together with those obtained in certain analogous 

 experiments by other investigators. This is done with a 

 view to ascertaining what they contribute to our knowl- 

 edge of the relative merits of the two more or less con- 

 tradictory hypotheses, multiple segregating factors or a 

 single fluctuating factor which are being advocated by 

 geneticists to explain certain cases of inheritance. 



It will be useful at the outset to state in a somewhat 

 definite manner how the alternative hypotheses differ from 

 one another. MacDowell (1916) in a recent paper has 

 clearly and precisely defined the two views. From this 

 start the following statement may be made. The first 

 view supposes that variations of the germ plasm are in the 

 nature of fluctuations. The germ plasm is in a continuous 

 state of variation. The hereditary characters all vary 

 under observation and this is taken to mean that the fac- 

 tors in the germ plasm determining them also vary. 

 Fluctuation in the character is measured and used as a 

 means of detecting and recording a similar though not 

 identical variation in the germinal factor underlying and 

 determining the character under observation. To use a 

 concrete example. In a given species the individuals are 

 of various sizes, some larger, some smaller. Some of this 

 variation is considered as in part due to non-heritable 

 environmental influences. There is, liowever, a distinct 

 "inheritance" of size. Tliis is considered tlie result of a 

 variable germinal factor \v!ii<'li a- (\'i-tle -uiz-est- may be 

 ^'Perhaps some substance or t'ernient wiii.-h varie< in 

 457 



