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. 
II. 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 factor-difference has a visible 
effect only at a certain age are in no fundamental 
respect different from cases like that of the Drosophila 
