34 



THE AMERICAN NATURALIST 



[Vol. LII 



are of importance: (1) F 1 between a non-shifting light 

 race and an always dark race is intermediate, or some- 

 what lighter in the beginning. But by progressive stages 

 the hybrid caterpillars shift over into the dark classes. 



Fig. 2 represents caterpillars from a cross of this type. 

 (The exact curves that belong with these pictures are re- 

 produced in our "Emfiihrung in die Vererbungswissen- 

 schaft," 2d edition, 1913, p. 170, Fig. 66, as an example 



