THEORY OF EVOLUTION 



51 



sooties to one ebony, putting the hybrids with 

 the sooties. The important fact to find out is 

 whether there actuallj^ exist three classes in the 



This can be ascertained 



second generation 



Fig. 20. Cross lietween two alleloniorphic races of Dro- 

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

 in F,. 



