112 THEORY OF EVOLUTION 



rounded; it may be longer than the body, or 

 shorter when other modifying factors are pres- 

 ent. The mutation that produces this type of 

 wing is of not infrequent occurrence. It has 

 been shown by JMuller and Altenburg that 

 tliere are at least two factors that modify this 

 character — the chief factor is present in the 

 second chromosome; alone it produces the 

 truncate wing in only a certain percentage of 

 cases, but when the modifiers are also j^resent 

 about ninety percent of the individuals may 

 show the truncate condition of the wing. But 

 the presence of these factors makes the stock 

 very infertile, so that it is difficult to maintain. 

 In tlie eighth figure (h) the legs are short- 

 ened owing to the absence of a segment of the 

 tarsus. The stock is called dachs — a nickname 

 given to it 

 dachsliund. 



given to it because the short legs suggested the 



Group III 



In figure 55, (a), a mutant type called bi- 

 thorax is shown. The old metathorax is re- 

 placed by another mesothorax thrust in between 

 the normal mesotliorax and tlie abdomen. It 



