THEORY OF EVOLUTION 109 



whether the parents had pads or expanded 

 wings. 



The flies of this stock show, however, another 

 character, which is a product of the same factor, 

 and which is constant, i.e., repeated in all in- 

 dividuals. The two bristles on the sides of 

 the thorax are constantly absent in this race. 

 The lighter color of the eye in the figure may 

 be taken to indicate buff a faint yellowish 

 color. The factor for this eye color is another 

 allelomorph of white. 



There are many other interesting characters 

 that belong to the first group, such as abnormal 

 abdomen, short legs, duplication of the legs, 

 etc. In fact, any part of the body may be af- 

 fected by a sex-linked factor. 



Group II 



In the first figure (a) of figure 54 that 

 contains members of Group II the wings are 

 almost entirely absent or "vestigial". This 

 condition arose at a single step and breeds 

 true, although it appears to be influenced to 

 some extent by temperature, also by modifiers 

 that sometimes appear in the stock. Purple 



