THE INFLUENCES OF NURTUEE 89 



A character known to be a part of the inheritance 

 may remain entirely unexpressed in the individual 

 development because certain environmental conditions 

 are lacking, yet the heritable character may be handed on 

 all the same. Let us consider the parable of the Fruit- 

 fly {Droso'pJiila ampelophila) as we find it in Professor 

 T. H. Morgan's book already quoted. There is a race 

 of these flies with a peculiar abnormality, and in ordinary 

 conditions the members of this abnormal race produce 

 others like themselves. But if the eggs of these flies 

 are reared in a very dry place the insects are all normal, 

 and one might be tempted to think that the vice had 

 vanished. But if the offspring of these apparently normal 

 flies are reared in a moist place, the abnormality is 

 again displayed. The point of the parable is missed if 

 it be thought that the wetness produces the abnormality 

 afresh. That is not the state of affairs. The hereditary 

 factor for the abnormality is there all the time, but it 

 finds expression in moist environment only. 



Another illustration from the Fruit-fly may be use- 

 ful. There is a divergent (or mutant) stock that pro- 

 duces supernumerary legs, in a considerable percentage 

 in winter, few or none in summer. Miss Hoge has 

 found that when the flies are kept in an ice-chest at 

 a temperature of about 10° C. there is a high percentage 

 of individuals with extra legs. 



In a hot climate there would be little or no evidence 

 that the peculiarity in question was part of the inheritance, 

 This shows that the expression of the inheritance as regards 

 a particular character sometimes depends on nurture. 



