HEREDITY 



the case of a pure line? We know the effects 

 should be less, but are they nil? Concerning 

 this matter we are perhaps justified in await- 

 ing further evidence. For in the case of beans 

 and of paramecium alike size is subject to very 

 great variation through the influence of nutri- 

 tion. Variations due to this cause are natu- 

 rally not inherited, since the germ-cells are not 

 affected by them, but only the body. But is 

 it not possible that along with the striking size 

 differences due to nutrition there may occur 

 also slight size differences due to germinal 

 variation within the pure line, that is owing 

 to variations in the potency of the same unit- 

 character or combination of unit-characters? 

 To be sure, Johannsen and Jennings have not 

 observed these, but this does not prove their 

 non-existence. Others may yet be able to do 

 so; indeed one case is already on record in 

 which such observations have been made in the 

 case of a small crustacean (or water-flea), 

 Daphnia. 



Daphnia is a small transparent animal, about 

 the size of a pin-head, which occurs in enor- 

 mous numbers in fresh-water lakes and pools, 



116 



