194 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 reés- 
tablish the old equilibrium of 24:1B. 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 codperation and 
codrdination 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. 
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 
ip their recent. papers on “Deficiency” and “‘An Oenothera-like case in Drosophila.” 
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