114 THEORY OF EVOLUTION 



ever, been separated by Bridges that are dif- 

 ferent in size as a result of a genetic factor. 

 The first of these, called dwarf, is represented 

 by figure 55, (b). 



The race is minute, although of course its 

 size is variable, depending on food and other 

 conditions. The same figure shows the pres- 

 ence of another factor, "sooty", that makes the 

 fly very dark. Maroon eye color might be 

 here represented, due to still another factor. 



In the third figure (c) the other mutation in 

 size is shown. It is called "giant". The flies 

 are twice the size of wild flies. An eye color, 

 called peach, might here be represented. It is 

 an allelomorph of pink. 



In the fourth figure (d) the mutant called 

 dichaete is shown. It is characterized by the ab- 

 sence of two of the bristles on the thorax. 

 Other bristles may also be absent, but not so 

 constantly as the two just mentioned. An- 

 other effect of the same factor is the spread-out 

 condition of the wings. The very dark eye 

 color in this figure may be taken to indicate 

 the presence of another factor, "sepia", which 

 causes the eyes to assume a brown color that 



