220 



T. H. MORGAN. 



has been formed either under domestication, or in Nature, not 



by one mutation. il >iep alone, hut bv a M-ries of succe^ive steps 



FIG. 



along the same line, that the crossing with the original type or 

 with a related type does not give Mendelian inheritance for one 



FIG. <>. 



pair of characters. What we should expert under such circuin- 

 stances \\ e do not know a- vet for \\ e are now dialing with a 



