THE FACTORIAL HYPOTHESIS 209 



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 



