70 THEORY OF EVOLUTION 



in which the wing pads did not expand (fig, 

 33). It was found that this pecuharity 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 rudimentarjr wings also produces other ef- 



