PAETICULATE THEOEY OF HEREDITY 239 



white-eyed condition. It follows that whatever it is in 

 the germ-plasm that produces white eyes, also produces 

 other modifications as well, and modifies not only such 

 "superficial" things as color, but also such '* fundamen- 

 tal ^ ' things as productivity and viability. Many examples 

 of this manifold effect are known to students of heredity. 

 It is perhaps not going too far to say that any change 

 in the germ-plasm may produce many kinds of effects on 

 the body. Clearly then the character that we choose to 

 follow in any case is only the most conspicuous or (for 

 purposes of identification) the most striking or convenient 

 modification that is produced. Since, however, these 

 effects always go together, and can be explained by the 

 assumption of a single unit difference in the germ-plasm, 

 the particular difference in the germ-plasm is more sig- 

 nificant than the character chosen as its index. 



2. The Variability of the Character is Not Due to the 



Corresponding Variability of the Gene 



All characters are variable, but there is at present 

 abundant evidence to show that much of this variability 

 is due to external conditions that the embryo encounters 

 during its development. Such differences as these are not 

 transmitted in kind — they remain only so long as the 

 environment that produces them remains. By inference 

 the gene itself is stable, although the character varies ; yet 

 this point is very difficult to establish. The evidence is 

 becoming stronger nevertheless that the germ-plasm is 

 relatively constant, while the character is variable. 



3. Characters That are Indistinguishable May be the 



Product of Different Genes 



We find, in experience, that we cannot safely infer 

 from the appearance of the character what gene is pro- 

 ducing it. There are at least three white races of fowls, 

 produced by different genes. We can synthesize white- 



