104 GENETICS IN RELATION TO AGRICULTURE 



of the behavior of certain chemical systems. It is a well-known fact 

 for instance that in some systems an equilibrium is reached when a certain 

 proportion of two substances are present in a chemical system. Thus a 

 system consisting of A and B, two compounds mutually convertible 

 into each other, may reach an equilibrium when say 2 parts of A and 1 

 of B are present in the system. If now a certain proportion of A is 

 removed from the system, enough of B will be converted into A to rees- 

 tablish the old equilibrium of 2A : IB. It is not difficult to see, there- 

 fore, that continuous removal of A from such a system would finally 

 result in the conversion of all of B into A. Assuming, therefore, that 

 our original system consisted merely of an unstable chemical compound, 

 it might be possible by continuously removing a certain product of its 

 instability to gradually alter the system in a given direction, much as we 

 have outlined the case for alteration of the hooded pattern by continuous 

 selection in rats. Since such changes are usually reversible, the efficacy 

 of return selection is adequately accounted for. 



Nevertheless, although it must be admitted that an interpretation 

 such as we have given above may account for all the known facts of 

 quantitative inheritance, and as the student can readily see it may be 

 employed to interpret the entire set of eight conditions which East 

 has outlined, we advocate the strict multiple factor hypothesis of size 

 inheritance for the following reasons: 



1. It is definitely known that large numbers of loci may be concerned 

 in the expression of a certain character. Morgan has stated that over 

 twenty-five factors are known to be concerned with eye color in Droso- 

 phila, and similarly a large number of factors affect body color and wing 

 characters. The assumption of large numbers of factors as concerned 

 with a single character does not, therefore, do violence to modern con- 

 ceptions of factor and character relationships. 



2. Size is a complex character depending on the cooperation and 

 coordination of many organs, tissues, and physiological processes. 

 Some factors may, therefore, affect one organ, some another, so from this 

 viewpoint a large number of factor differences might be expected to be 

 present in cases of quantitative inheritance. 



3. Although factor constancy cannot yet be considered a universally 

 established fact, those definite investigations which have been reported 

 indicate that factors possess on the whole a high degree of stability. 

 More definite work is needed along this line; provisionally it appears 

 wise to consider factors for all practical purposes as constant. 1 



4. Simple factor differences are known to give size differences, 



1 That factors are relatively stable entities is being evidenced more clearly all the 

 time. Witness the definite arguments advanced by Bridges and Muller respectively 

 in their recent, papers on "Deficiency" and "An Oenothera-like case in Drosophila." 



