THEORY OF EVOLUTION 117 



tant is likewise very variable, often indistin- 

 guishable from the wild type, yet when well 

 developed strikingly different from any other 

 mutant. 



This brief account of a few of the mutant 

 races that can be most easily represented by 

 uncolored figures will serve to show how all 

 parts of the body may change, some of the 

 changes being so slight that they would be 

 overlooked except by an expert, others so great 

 that in the character affected the flies depart 

 far from the original species. 



It is important to note that mutations in the 

 first chromosome are not limited to any part 

 of the body nor do they affect more frequently 

 a particular part. The same statement holds 

 equally for all of the other chromosomes. In 

 fact, since each factor may affect visibly sev- 

 eral parts of the body at the same time there 

 are no grounds for expecting any special rela- 

 tion between a given chromosome and special 

 regions of the body. It can not too insistently 

 be urged that when we say a character is the 

 product of a particular factor we mean no 

 more than that it is the most conspicuous effect 

 of the factor. 



