518 



THE AMERICAN NATURALIST 



[Vol. LI 



the body. Clearly then the character that we choose to 

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

 us) the most 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, this particular element or gene in the 

 germ plasm is more significant than the character chosen 

 as an index for one only of the effects. 



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 abun- 

 dant evidence to show that much of this variability is 

 due to the 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 tlioui roMiains. By inference 

 the gene itself is staiilc altlimioli the character varies; 

 yet this point is very dimciilt to e>tablish. The evidence 

 is becoming stronger nevertheless that the germ plasm 

 is relatively constant, while the character is variable. I 

 shall consider this evidence in another connection. Here 

 I wish merely to register some of the reasons why the 

 idea of the gene is useful. 



3. Characters that <irr I i/di^fnif/idshable may be the 

 Product of Different Genes 



AVe find, in experience, that we can not 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- 

 eyed flies that are somatically indistinguishable from the 

 ordinary white-eyed race, yet they are the combined prod- 

 uct of several kuowu licues. Tlie purple eye color of 

 Drosnpii'ilii ]M-artica!!\ iiidistiiiu-uishable from the eye 

 col(n's maroon and uariieT. In a vvoi'd we are led again to 

 units iu the germ i>hi>m in our final analysis rather than 

 to the appearance of a character. 



