THEORY OF EVOLUTION 



51 



sooties to one ebony, putting the hybrids with 

 the sooties. The important fact to find out is 

 whether there actually exist three classes in the 

 second generation. This can be ascertained 



' 



FIG. 20. Cross between two allelomorphic races of Dro- 

 sophila, sooty and ebony, that give a completely graded series 

 in F,. 



