THE FACTORIAL HYPOTHESIS 263 
One can therefore easily imagine that when one of 
these 25 factors changes, a different end result is 
produced, such as pink eyes, or vermilion eyes, or 
white eyes or eosin eyes. Each such color may be 
the product of 25 factors (probably of many more) 
and each set of 25 or more differs from the normal 
in a different factor. It is this one different factor 
that we regard as the “unit factor” for this particular 
effect, but obviously it is only one of the 25 unit 
factors that are producing the effect. However'since 
it is only this one factor and not all 25 which causes 
the difference between this particular eye color and 
the normal, we get simple Mendelian segregation in 
respect to this difference. In this sense we may say 
that a particular factor (p) is the cause of pink, for we 
use cause here in the sense in which science always 
uses this expression, namely, to mean that a particu- 
lar system differs from another system only in one 
special factor. 
The converse relation is also true, namely, that a 
single factor may affect more than one character. 
For example, the factor for rudimentary wings in 
Drosophila affects not only the wings, but the legs, 
the number of eggs laid, the viability, etc. Indeed, 
in his definition of mutation, DeVries supposed that a 
change in a unit factor involves all parts of the body. 
The germ cells may be thought of as a mixture of 
many chemical substances, some of them more closely 
related to the production of a special character, color, 
for example, than are others. If any one of the sub- 
stances undergoes a change, however ‘slight, the end 
