THE AMERICAN NATURALIST [Vol. XLVI 



tion has therefore not been materially affected, but a 

 progressive change has been made in the mode about 

 which variation takes place. 



3. The plus and minus series have from time to time 

 been crossed with the same wild race. Each behaves as 

 a simple recessive unit giving a 3:1 ratio among the 

 grandchildren. But the extracted plus and the extracted 

 minus individuals are different ; the former are the more 

 extensively pigmented. 



4. The series of animals studied is large enough to 

 have significance. It includes more than 10,000 individ- 

 uals. 



The conclusion seems to me unavoidable that in this 

 case selection has modified steadily and permanently a 

 character unmistakably behaving as a simple Mendelian 

 unit. 



In my experience every unit-character is subject to 

 quantitative variation, that is, its expression in the body 

 varies, and it is clear that these variations have a ger- 

 minal basis because they are inherited. By selection 

 plus or minus through a series of generations we can in- 

 tensify or diminish the expression of a character, that is, 

 we can modify the character. 



In an earlier lecture I showed that long hair and rough 

 coat in guinea-pigs each differ from the normal condi- 

 tion by a single unit-character. In 1906 I showed that 

 both these characters are subject to quantitative varia- 

 tion, and that such variations are heritable. The same 

 is true of polydactylism in guinea-pigs, a condition in 

 which a fourth toe is present on the hind-foot. A poly- 

 dactylous race of guinea-pigs was unknown until I cre- 

 ated one by selection from among the descendants of a 

 single abnormal individual which had a rudimentary 

 fourth-toe on one hind-foot. For several generations in 

 succession those individuals were selected for parents 

 which had the best-developed extra-toe, and thus was 

 obtained a good 4-toed race. 



Another character built up slowly from small begin- 

 nings is the silvered variety of guinea-pig. It originated 



