ORGANIC EVOLUTION 21 



at the other. These types, with the fluctuations that 

 occur within each tyj)e, furnish a complete series 

 of gradations ; vet historically they have arisen inde- 

 pendently of each other. 



JNIany changes in eye color have appeared. As 



* 



'« 



•SLi^ 



fl 





"msg^i^^ 



A 



-^-m 



**% 



^M^: 



' d 



¥ 



-^^*EV- 



FiG. 7. — Mutants of Drosophila melanogaster, arranged in 

 order of size of wings; rr, cut,; 6, beaded; c, stumpy; c/, an- 

 other stumpy; e, vestigial; j, apterous. 



many as fifty or more races differing in eye color are 

 now maintained in our cultiu'cs. Some of them are 

 so similar that they can scarcely be separated from 

 each other. It is easily possible beginning with the 

 darkest eye color, sepia, which is deep brown, to 

 2)ick out a perfectly graded series ending with pure 

 white eyes. But such a serial arrangement would 



