70 THEORY OF EVOLUTION 



in which the wing pads did not expand (fig. 

 33). It was found that this peculiarity is 

 shown in only about twenty per cent of the in- 

 dividuals supposed to inherit it. Later it was 

 found that this stock lacked two bristles on the 

 sides of the thorax. By means of this knowl- 

 edge the heredity of the character was easily 

 determined. It appears that while the expan- 

 sion of the wing pads fails to occur once in five 

 times probably because it is an environmental 

 effect peculiar to this stock, yet the minute 

 difference of the presence or absence of the two 

 lateral bristles is a constant feature of the flies 

 that carry this particular factor. 



In the preceding cases I have spoken as 

 though a factor influenced only one part of the 

 body. It would have been more accurate to 

 have stated that the chief effect of the factor 

 was observed in a particular part of the body. 

 Most students of genetics realize that a factor 

 difference usually affects more than a single 

 character. For example, a mutant stock called 

 rudimentary wings has as its principle character- 

 istic very short wings (fig. 34) . But the factor 

 for rudimentary wings also produces other ef- 



