42 TYPES OF MENDELIAN HEREDITY 



obscured. In a hot climate there would be no evi- 

 dence that such a factor was being regularly trans- 

 mitted. But if the type moved into a cold region 

 it would show duplication in many of the legs. 



//. By Developmental Influences 



''Age," too, is in a sense an environmental condi- 

 tion, which influences the development of ^characters. 

 Thus a white flower may change to purple as the plant 

 gets older, or the flaxen hair of a child may turn to 

 brown when he becomes a man. But, as in the case 

 of other "environmental" conditions, age may not 

 have the same effect on individuals with different 

 factors; in this way it comes about that animals or 

 plants which differ by certain factors may show a 

 difference in character only at certain ages, or may 

 not show the same difference at all ages. In Droso- 

 phila, flies with the factor for pink eyes are easily 

 distinguishable from those with the factor for purple 

 eyes, when the flies are young, but as they grow older, 

 the eyes of both races assume a dark purplish shade, 

 and become practically indistinguishable from each 

 other. Conversely, old flies with the factor for black 

 are usually easy to separate from those having the 

 normal "gray" factor, but the newly hatched flies, 

 in which the black pigment is not yet fully developed, 

 are separated with greater difficulty. 



These cases in which a f actor-difTerence has a visible 

 effect only at a certain age are in no fundamental 

 respect different from cases like that of the Drosophila 



