THEORY OF EVOLUTION 



69 



The preceding cases have all involved rather 

 great changes in some one organ of the body. 

 The following three cases involve slight 

 changes, and yet follow the same laws of inheri- 

 tance as do the larger changes. 



At the base of the wings a minute black 

 speck appeared (fig. 32). It was found to 

 be a Mendelian character. In another case the 

 spines on the thorax became forked or kinky 



FIG. 33. Mutant race of fruit fly called club. The wings 

 often remain unexpanded and two bristles present in wild 

 fly (b) are absent on side of thorax (c). 



(fig. 53b). This stock breeds true, and the 

 character is inherited in strictly Mendelian 

 fashion. 



In a certain stock a number of flies appeared 



